show invisible key change events

[yuxshao]

If there's a key change event that either doesn't happen when a note is played, or is the same key as the previous note, it won't show up. This can be sorta confusing, so probably:

1. Key changes that happen in the middle of the note could have a 1-pixel gap. Maybe
2. Add some other visual indicator for key changes (maybe a little white triangle? Or the skinny line like in collage for the current unit)