Change FX tab to use SliderButtons, instead of Knobs

Knob UI is too tricky to get perfect right now. This is a little less
pretty, but a bit more functional
Abstract out button updating when oscillator settings change
Properly vertically align EqPanel

Gotta take half the height plus the top value in order to get the right
value
Have all drawable elements calculate text size on setting dimensions
Show current instrument name on Synth Manager tab