Bar UI mechanics pretty much done. Still need to:
- Hook up buttons
- Position UI within parent bounds (done for x axis, see next point for y axis)
- Handle cases where lines are very small
- Make UI fade out when playing starts or mouse stops for a while

todo
- Fix race condition around PlayAndRemoveNoteOffs (called from Pause command on UI thread and HandleTick on timer thread). Check for other race conditions.
- Try/catch in HandleTick (especially for when app is closed down when playing) and other events/properties.
- Find/fix note ons not turning off.
- Find or make themes for numericUpDown control (or find another control).
- Fix bug in numericUpDown max value going too far.
- Algorithm for determining left/right hand, and UI to change it.

Fixed problems in generic numericupdown theme; it'll do for now.