PianoRoll: CSS stylability

Most parts of piano roll are now stylable:
- background color
- note color
- bar color (for volume bars)
- grid color
- color of the note volume/note panning text 

Some are still unstylable: selection colour, pitch/scale highlight colours. I think the selection colour would be best if it could be defined globally in one property, but I don't yet know how to go about implementing this...