Fix typo.
Fix potential pop of empty enter stack.
Clean up note delta command code.

Use Variant to store the value and the same code path for all properties.

Factor out getting the value of whatever property instead of special casing the
handling.

Towards using this stuff for some fancy things...
Remove pointless register specifier and warning.
Fix occasional MIDI read crash.

Locking should prevent this from being a problem, but taking a reference to the
cached iterator and mutating it directly causes occasional crashes for me for
reasons I can't quite figure out.

This fixes the issue and is arguably more sane anyway, so whatever.
MIDI transform dialog.
Update note colors when model changes.