CSS styling features, Monstro updates

Allows linear gradients for the track background. Both darker/lighter squares can be set separately with 3 user-definable colours. The gradient midpoint can also be set.

I think this is probably the best compromise between simplicity and flexibility.

Addition: patterns' foreground elements are now also stylable. The text of the pattern names, and the foreground colour (notes in instrument tracks, wavegraph in sample tracks, automation curve in automation tracks) are both defined in CSS now.

Addition: some Monstro updates.
- fix some bugs with phase handling
- optimize sync codepaths
- add some amplitude delta caps as primitive means of aliasing reduction (not sure how effective this is, but I didn't want to do anything more complex since we're already using quite a bit of cpu in some cases)
- also increase some of the envelope maximum amounts
- also modify spo knob behaviour to be more sensible