Add support for parentheses before/after chords.

This allows a chord or sequence of chords to be indicated as optional
or alternative, and avoids the need to add the parentheses as text
or extra chords.
Remove obsolete assignment to _userName
Some refactoring, more comments
Tree view for MIDI import operations
Refactoring of quantization
Meter-adapted duration subdivision
Row for all tracks selection in GUI panel
Adapt tst_importmidi to new duration subdivision
fix #21246 reduce text cursor width
fix #21223
fix #21243 allow multiple selection in instrument list
fix #21235 add bracket symbol to notehead master palette
fix #21233 key signature spacing
space flats in key signature closer
better multi selection in instrument dialog
rollback keysig layout changes
fix #21218
fix #21249
fix tick of grace note after note
fix #6535 : Impossible to tie to a grace note