Refactor PianoRoll::getSelectedNotes()

Currently, PianoRoll::getSelectedNotes loops over all notes to find the ones where isSelected is true, adds them to a NoteVector, and returns that. This is done every time the function is called, which is not efficient. @szeli1 brought this to my attention in #7759.

This is mostly a TODO issue to remind me to do this, but if anyone else wants to try to tackle it, feel free!

Implementation Ideas

• The selected notes should probably be cached in some kind of vector.
• Since the piano roll is afaik the only class which actually interacts with the isSelected state of the notes, perhaps for all the occurances of a note being selected/deslected, we could add a line of code which adds/removes that note from the vector. We would have to be very careful to address all the occurances, and also repopulate the vector whenever a new clip is opened in the piano roll (Since selections of notes persist within each MidiClip; selecting some notes in one clip, opening another clip, and opening the old clip will retain the selection).
• Or, we could implement that logic within Note::setSelected. However, this would probably require either giving Note access to the piano roll or having some kind of static vector, which may not be desirable.

Start with PianoRoll::getSelectedNotes and trace every place where notes are selected or deselected, including switching between MidiClips. Review Note::setSelected and the existing PianoRoll selection flow before choosing an approach. Done means selected-note results stay correct across selection changes and clip changes without scanning all notes on every call.