rework pixfader:

* re-introduce static pattern cache
  (cairo's cache is not nearly large enough for A3 + plugins)
* only use cairo_clip on rectangles (not arbitrary paths
  -> major performance boost
* re-add fader pixel reserve
* fix mouse + scroll interaction
* fix unity line display (1px wider, brighten by 150%)
* cache text size
* fix various other issues and consistent whitespace

This reverts large parts of commit d439e93b1e71e84452bf52ddc69ff0be6aa04ef5.
Engine Dialog: focus OK button (press enter to cont w/previous settings)
prepare Slider API cleanup
remove cruft from SliderController
clean up pixfader API, prepare for merge w/BarController
unify BarController with Slider/Pixfader.
add style for BarController SpinButton
fix static initialization of Gtk::SizeGroup