fix ever increasing MIDI event IDs

Iterating over a const Midi-Sequence calls Evoral::Sequence::set_event(),
which in turn used Evoral::Event::operator=()  which always created
a new event-ID (create copy of the event).

Issues fixed:
 - Saving *unmodified* MIDI produced new event-IDs on every save;
   files changed with every save. - greetings to Deva.
 - all [GUI] operations that use IDs to refer to notes e.g. undo.
   invalid undo-history.

Also clarify assignment operator name. Prefer explicit assign() over =.

1 files changed, 1 insertions, 1 deletions

diff --git a/libs/evoral/evoral/Event.hpp b/libs/evoral/evoral/Event.hpp
index ec92d575d0..2a6810a3b4 100644
--- a/libs/evoral/evoral/Event.hpp
+++ b/libs/evoral/evoral/Event.hpp
@@ -61,7 +61,7 @@ public:
 
         ~Event();
 
-	const Event& operator=(const Event& copy);
+	void assign (const Event& other);
 
 	void set(const uint8_t* buf, uint32_t size, Time t);