add velocity editor and asset controls to piano roll

[riknoll]

more work on the piano roll editor!

changes:

• added the usual asset editor chrome (name, done button, editor/my assets toggle)
• added velocity support for songs (more on this below)
• added a velocity editor to the piano roll
• cleaned up a lot of css
• made it blue
• fixed parsing of PXT songs

velocity support involves a change to the song format, but it's backwards compatible. the new data is simply tacked on to the end of the song buffer after all of the track data (and is completely optional). this new format will also work just fine with the old player code; the velocity values will just get ignored.

it should be completely safe to open a song from the old song editor in the piano roll editor, but going the other direction might cause you to lose velocity data so it's not recommended. i might add a warning dialog at some point to prevent this from accidentally happening.

i will eventually have another PR in common-packages that adds velocity support to the hardware sequencer implementation. again, it'll be 100% backwards compatible with existing songs.