Midi-player

- Add `loop` attribute
- Add `reload()` method to player and visualizer
- When assigning to `noteSequence`, do not reload it if it was already loaded (if the sequence was changed in place, `reload()` now needs to be called)
- This also means the visualizer now won't reload when restarting playback
- Reduce some stuttering at playback start by reloading the visualizers first

- Add a loading shimmer effect
- Add an error icon and tooltip

- Add a scroll bar to the piano roll visualizer
- Make `<midi-visualizer>` a block element to force it to have dimensions
- Make `<midi-visualizer>` use shadow DOM, but put the actual visualizer in a slot so that it can still be easily styled with CSS
- Import `magenta/music` as ES modules

- Fixed behavior after playback finishes