refactor: replace Song::modeNotes and numModeNotes with a NoteSet

[nikodemus]

• NoteSet::operator[] gets note value of a scale degree.
• NoteSet::fromScaleNotes() constructs a NoteSet from scale notes.
• NoteSet::applyChanges() applies an array of scale degree semitone offsets.
• NoteSet::addUntrusted() implements safeguards that loading from song file uses.
• NoteSet::clear() clears a set.
• NoteSet::highest() returns the highest note in the set.

[github-actions[bot]]

Test Results

56 tests  +8   56 :white_check_mark: +8   0s :stopwatch: ±0s  9 suites ±0    0 :zzz: ±0   9 files   ±0    0 :x: ±0 

Results for commit 75dff701. ± Comparison against base commit 99281139.