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 =.
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