fix ever increasing MIDI event IDs

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 =.