UI changes made so far

The UI project is now in a state where I feel like I can request the changes so far to be merged. This would be good so that the fork/branch doesn't diverge too much from upstream which might cause problems later.

All kinds of neat stuff here: new leds by Tobiasz, my sf2 artwork, stylesheet changes, customizable palette, fx mixer fxline highlighting, some smaller tweaks, making the UI more customizable...