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, 43 insertions, 3 deletions

diff --git a/gtk2_ardour/stereo_panner.cc b/gtk2_ardour/stereo_panner.cc
index 721567bf10..c2b38867ab 100644
--- a/gtk2_ardour/stereo_panner.cc
+++ b/gtk2_ardour/stereo_panner.cc
@@ -34,6 +34,7 @@
 
 #include "ardour/pannable.h"
 #include "ardour/panner.h"
+#include "ardour/panner_shell.h"
 
 #include "ardour_ui.h"
 #include "global_signals.h"
@@ -58,8 +59,9 @@ bool StereoPanner::have_colors = false;
 
 using namespace ARDOUR;
 
-StereoPanner::StereoPanner (boost::shared_ptr<Panner> panner)
-	: PannerInterface (panner)
+StereoPanner::StereoPanner (boost::shared_ptr<PannerShell> p)
+	: PannerInterface (p->panner())
+	, _panner_shell (p)
 	, position_control (_panner->pannable()->pan_azimuth_control)
 	, width_control (_panner->pannable()->pan_width_control)
 	, dragging_position (false)
@@ -80,6 +82,7 @@ StereoPanner::StereoPanner (boost::shared_ptr<Panner> panner)
 
 	position_control->Changed.connect (connections, invalidator(*this), boost::bind (&StereoPanner::value_change, this), gui_context());
 	width_control->Changed.connect (connections, invalidator(*this), boost::bind (&StereoPanner::value_change, this), gui_context());
+	_panner_shell->Changed.connect (connections, invalidator (*this), boost::bind (&StereoPanner::bypass_handler, this), gui_context());
 
 	ColorsChanged.connect (sigc::mem_fun (*this, &StereoPanner::color_handler));
 
@@ -94,6 +97,10 @@ StereoPanner::~StereoPanner ()
 void
 StereoPanner::set_tooltip ()
 {
+	if (_panner_shell->bypassed()) {
+		_tooltip.set_tip (_("bypassed"));
+		return;
+	}
 	double pos = position_control->get_value(); // 0..1
 
 	/* We show the position of the center of the image relative to the left & right.
@@ -145,7 +152,11 @@ StereoPanner::on_expose_event (GdkEventExpose*)
 
 	/* background */
 
-	context->set_source_rgba (UINT_RGBA_R_FLT(b), UINT_RGBA_G_FLT(b), UINT_RGBA_B_FLT(b), UINT_RGBA_A_FLT(b));
+	if (!_panner_shell->bypassed()) {
+		context->set_source_rgba (UINT_RGBA_R_FLT(b), UINT_RGBA_G_FLT(b), UINT_RGBA_B_FLT(b), UINT_RGBA_A_FLT(b));
+	} else {
+		context->set_source_rgba (0.1, 0.1, 0.1, 0.2);
+	}
 	cairo_rectangle (context->cobj(), 0, 0, width, height);
 	context->fill ();
 
@@ -183,6 +194,10 @@ StereoPanner::on_expose_event (GdkEventExpose*)
 	context->set_source_rgba (UINT_RGBA_R_FLT(r), UINT_RGBA_G_FLT(r), UINT_RGBA_B_FLT(r), UINT_RGBA_A_FLT(r));
 	context->stroke ();
 
+	if (_panner_shell->bypassed()) {
+		return true;
+	}
+
 	/* compute & draw the line through the box */
 
 	context->set_line_width (2);
@@ -267,6 +282,10 @@ StereoPanner::on_button_press_event (GdkEventButton* ev)
 	if (PannerInterface::on_button_press_event (ev)) {
 		return true;
 	}
+
+	if (_panner_shell->bypassed()) {
+		return true;
+	}
 	
 	drag_start_x = ev->x;
 	last_drag_x = ev->x;
@@ -412,6 +431,10 @@ StereoPanner::on_button_release_event (GdkEventButton* ev)
 		return false;
 	}
 
+	if (_panner_shell->bypassed()) {
+		return false;
+	}
+
 	bool const dp = dragging_position;
 
 	_dragging = false;
@@ -443,6 +466,10 @@ StereoPanner::on_scroll_event (GdkEventScroll* ev)
 	double wv = width_control->get_value(); // 0..1.0 ; 0 = left
 	double step;
 
+	if (_panner_shell->bypassed()) {
+		return false;
+	}
+
 	if (Keyboard::modifier_state_contains (ev->state, Keyboard::PrimaryModifier)) {
 		step = one_degree;
 	} else {
@@ -474,6 +501,9 @@ StereoPanner::on_scroll_event (GdkEventScroll* ev)
 bool
 StereoPanner::on_motion_notify_event (GdkEventMotion* ev)
 {
+	if (_panner_shell->bypassed()) {
+		_dragging = false;
+	}
 	if (!_dragging) {
 		return false;
 	}
@@ -566,6 +596,10 @@ StereoPanner::on_key_press_event (GdkEventKey* ev)
 	double wv = width_control->get_value(); // 0..1.0 ; 0 = left
 	double step;
 
+	if (_panner_shell->bypassed()) {
+		return false;
+	}
+
 	if (Keyboard::modifier_state_contains (ev->state, Keyboard::PrimaryModifier)) {
 		step = one_degree;
 	} else {
@@ -641,6 +675,12 @@ StereoPanner::color_handler ()
 	queue_draw ();
 }
 
+void
+StereoPanner::bypass_handler ()
+{
+	queue_draw ();
+}
+
 PannerEditor*
 StereoPanner::editor ()
 {