given beat can have no more than one StopSegment or BPMSegment--more
than one just doesn't make sense. Split out TimingData::InsertRows,
TimingData::DeleteRows; delete rows in between before shifting data
up.
move sync display/saving out of complicated ScreenGameplay
show sync UI in editor playback as well as gameplay
revert sync from in-memory backup, not from disk
move more functions into debug overlay
Maybe we should stop calling note indexes "rows" and "indexes", and just
call them "beats"; if they're stored as an integer, they're in fixed-point. Things
like "note rows per second" are a lot less intuitive than just calling them "beats
per second".