Suppress midi in events on export

[zonkmachine]

Addresses #3747. This suppresses midi events (sustain pedal, key press) triggered during export. It does however not clear out midi events in effect while starting the export.

[PhysSong]

If sustin pedal have been pressed before export, it will be exported. Before rendering, all notes should be silenced and sustain pedal should be marked as released.

[zonkmachine]

If sustin pedal have been pressed before export, it will be exported.

Which this PR doesn't take care of. It's a partial fix as mentioned here. I don't have time to dive into the issue more.

[zonkmachine]

I'm merging this tomorrow if there are no other complaints. #3747 isn't marked for 1.2.0 and shouldn't be. This PR will make the issue less prominent.

[PhysSong]

I'm going to test this after my works for rc4 finish. If I find any regressions, I'll request changes. Otherwise, it's good:+1:

[PhysSong]

Tested. Just a note:
If you press keys/sustain pedal before exporting, there are no ways to correct during the export. It doesn't matter much since those keys/pedals breaks output even they can be canceled.
Such things should be handled somehow, but it can be done later.

[zonkmachine]

Tested. Just a note:
If you press keys/sustain pedal before exporting, there are no ways to correct during the export.

OK. I think I've already commented on this.

Merging 🐳