rework panning -- Squashed commit of the following:

commit 6f4f4f161b00cb36252727f67ecc4913eb944fd7
Author: Robin Gareus <robin@gareus.org>
Date:   Wed Jan 8 22:13:09 2014 +0100

    fix panner plugin discovery (prev commit)

commit 26e514f4a80af9192cae3cbd62fde0ae95474dfc
Author: Robin Gareus <robin@gareus.org>
Date:   Wed Jan 8 18:56:59 2014 +0100

    update panner plugin discovery

    * recurse dirs in 'PANNER_PATH' and 'panner_dir_name' up to 1 level.
    * don't look in ardour_dll_directory() -- no panners are supposed to be in there
    * use .dylib on OSX exclusively.

commit a514c3f1c425dccf3d42eee9d2b183b44fd26a03
Author: Robin Gareus <robin@gareus.org>
Date:   Wed Jan 8 16:48:34 2014 +0100

    remove debug/devel printf()s

commit d863742ddc69af493ee6a8817bc778968d9b0800
Author: Robin Gareus <robin@gareus.org>
Date:   Wed Jan 8 16:17:13 2014 +0100

    panner-type: session backward/forward compatibility

commit 25d5e4c663ada34129451b0f9045ab047d6cc2f0
Author: Robin Gareus <robin@gareus.org>
Date:   Wed Jan 8 16:09:07 2014 +0100

    update URIs -> URLs

commit 00a606a43d9456cfbaf43cae4fb598549326ba71
Merge: 0f1cec1 382eb0f
Author: Robin Gareus <robin@gareus.org>
Date:   Wed Jan 8 03:29:45 2014 +0100

    Merge branch 'master' into panning

commit 0f1cec19babae538c9697eed4be5d6ddc851b013
Author: Robin Gareus <robin@gareus.org>
Date:   Wed Jan 8 02:41:15 2014 +0100

    switch panner ID to URI

commit 575282b412c3ae1cd8219cf75f00a1a4239e2813
Author: Robin Gareus <robin@gareus.org>
Date:   Wed Jan 8 00:50:15 2014 +0100

    prepare API for panner URI

commit ea62cd049308859782a7bb16e4f18169d8638b46
Author: Robin Gareus <robin@gareus.org>
Date:   Tue Jan 7 19:57:06 2014 +0100

    update development doc relating to panner selection

commit 586d7de2392e26b9d7f597b1a00b98dfaa42ecdc
Author: Robin Gareus <robin@gareus.org>
Date:   Tue Jan 7 19:56:24 2014 +0100

    clean up PanShell::set_user_selected_panner_type() API

commit 99077886a5a1cacece908d87c29c3be12903027e
Author: Robin Gareus <robin@gareus.org>
Date:   Tue Jan 7 04:46:22 2014 +0100

    panner bypass: visualize & [in]sensitivity

commit 46d688d216f0e67d672376a607157af02b359fb2
Merge: 4e67573 c4cdf61
Author: Robin Gareus <robin@gareus.org>
Date:   Tue Jan 7 02:18:54 2014 +0100

    Merge branch 'master' into panning

commit 4e67573517b3d60ddf65729783687b16cfb2adb7
Author: Robin Gareus <robin@gareus.org>
Date:   Tue Jan 7 01:05:17 2014 +0100

    don't call configure_io() for merely swapping panners

commit d32a4c51f6967f48f7680554866f1f7b311ccde1
Merge: a3226d4 cec3116
Author: Robin Gareus <robin@gareus.org>
Date:   Mon Jan 6 23:49:55 2014 +0100

    Merge branch 'master' into panning

commit a3226d46b598afae54a65ac69320eca84669f347
Author: Robin Gareus <robin@gareus.org>
Date:   Mon Jan 6 17:52:38 2014 +0100

    add notes about panner re-design

commit d1ae2366024605f22b05572a81ee249e6fdbcd2f
Author: Robin Gareus <robin@gareus.org>
Date:   Mon Jan 6 15:06:40 2014 +0100

    add simple stereo-balance panner for testing

commit e0ddd256ff2288b8d8cfad3ad485a916964ce5b5
Author: Robin Gareus <robin@gareus.org>
Date:   Mon Jan 6 17:02:52 2014 +0100

    add frontend/GUI for panner selection

commit 2cb8f846755eb5aea8a2620d31ea981c446c4041
Author: Robin Gareus <robin@gareus.org>
Date:   Mon Jan 6 17:02:20 2014 +0100

    prepare backend for panner selection

1 files changed, 48 insertions, 2 deletions

diff --git a/libs/ardour/panner_shell.cc b/libs/ardour/panner_shell.cc
index 1aeb94eb7e..a25cb49ab5 100644
--- a/libs/ardour/panner_shell.cc
+++ b/libs/ardour/panner_shell.cc
@@ -63,6 +63,10 @@ PannerShell::PannerShell (string name, Session& s, boost::shared_ptr<Pannable> p
 	: SessionObject (s, name)
 	, _pannable (p)
 	, _bypassed (false)
+	, _current_panner_uri("")
+	, _user_selected_panner_uri("")
+	, _panner_gui_uri("")
+	, _force_reselect (false)
 {
 	set_name (name);
 }
@@ -82,7 +86,7 @@ PannerShell::configure_io (ChanCount in, ChanCount out)
 	   the config hasn't changed, we're done.
 	*/
 
-	if (_panner && (_panner->in().n_audio() == nins) && (_panner->out().n_audio() == nouts)) {
+	if (!_force_reselect && _panner && (_panner->in().n_audio() == nins) && (_panner->out().n_audio() == nouts)) {
 		return;
 	}
 
@@ -90,17 +94,21 @@ PannerShell::configure_io (ChanCount in, ChanCount out)
 		/* no need for panning with less than 2 outputs or no inputs */
 		if (_panner) {
 			_panner.reset ();
+			_current_panner_uri = "";
+			_panner_gui_uri = "";
 			Changed (); /* EMIT SIGNAL */
 		}
 		return;
 	}
 
-	PannerInfo* pi = PannerManager::instance().select_panner (in, out);
+	PannerInfo* pi = PannerManager::instance().select_panner (in, out, _user_selected_panner_uri);
 	if (!pi) {
 		cerr << "No panner found: check that panners are being discovered correctly during startup.\n";
 		assert (pi);
 	}
 
+	DEBUG_TRACE (DEBUG::Panning, string_compose (_("select panner: %1\n"), pi->descriptor.name.c_str()));
+
 	boost::shared_ptr<Speakers> speakers = _session.get_speakers ();
 
 	if (nouts != speakers->size()) {
@@ -116,6 +124,8 @@ PannerShell::configure_io (ChanCount in, ChanCount out)
 	// boost_debug_shared_ptr_mark_interesting (p, "Panner");
 	_panner.reset (p);
 	_panner->configure_io (in, out);
+	_current_panner_uri = pi->descriptor.panner_uri;
+	_panner_gui_uri = pi->descriptor.gui_uri;
 
 	Changed (); /* EMIT SIGNAL */
 }
@@ -126,6 +136,7 @@ PannerShell::get_state ()
 	XMLNode* node = new XMLNode ("PannerShell");
 
 	node->add_property (X_("bypassed"), _bypassed ? X_("yes") : X_("no"));
+	node->add_property (X_("user-panner"), _user_selected_panner_uri);
 
 	if (_panner) {
 		node->add_child_nocopy (_panner->get_state ());
@@ -146,12 +157,29 @@ PannerShell::set_state (const XMLNode& node, int version)
 		set_bypassed (string_is_affirmative (prop->value ()));
 	}
 
+	if ((prop = node.property (X_("user-panner"))) != 0) {
+		_user_selected_panner_uri = prop->value ();
+	}
+
 	_panner.reset ();
 	
 	for (niter = nlist.begin(); niter != nlist.end(); ++niter) {
 
 		if ((*niter)->name() == X_("Panner")) {
 
+			if ((prop = (*niter)->property (X_("uri")))) {
+				PannerInfo* p = PannerManager::instance().get_by_uri(prop->value());
+				if (p) {
+					_panner.reset (p->descriptor.factory (_pannable, _session.get_speakers ()));
+					_current_panner_uri = p->descriptor.panner_uri;
+					_panner_gui_uri = p->descriptor.gui_uri;
+					if (_panner->set_state (**niter, version) == 0) {
+						return -1;
+					}
+				}
+			}
+
+			else /* backwards compatibility */
 			if ((prop = (*niter)->property (X_("type")))) {
 
 				list<PannerInfo*>::iterator p;
@@ -166,6 +194,8 @@ PannerShell::set_state (const XMLNode& node, int version)
 						*/
 
 						_panner.reset ((*p)->descriptor.factory (_pannable, _session.get_speakers ()));
+						_current_panner_uri = (*p)->descriptor.panner_uri;
+						_panner_gui_uri = (*p)->descriptor.gui_uri;
 
 						if (_panner->set_state (**niter, version) == 0) {
 							return -1;
@@ -347,3 +377,19 @@ PannerShell::bypassed () const
 {
 	return _bypassed;
 }
+
+/* set custom-panner config
+ *
+ * This function is intended to be only called from
+ * Route::set_custom_panner()
+ * which will trigger IO-reconfigutaion if this fn return true
+ */
+bool
+PannerShell::set_user_selected_panner_uri (std::string const uri)
+{
+	if (uri == _user_selected_panner_uri) return false;
+	_user_selected_panner_uri = uri;
+	if (uri == _current_panner_uri) return false;
+	_force_reselect = true;
+	return true;
+}