Stability and fixes

This update focuses on reliability and polish.

- Restore Purchase now reliably recognizes an existing purchase
- Time signatures no longer overlap in some notation fonts
- Chord names keep their accidentals, inversions, and symbols after reopening
- Clef octave markings are preserved when you save and reopen
- More reliable PDF, audio, Guitar Pro, MusicXML, and MIDI export
- Fixed several rare crashes