- derive it from GtkScale not GtkRange so we get the keybindings again
- remove scroll_event() implementation, the parent class does the right
  thing now
- remove lots of magic size calculation code about focus and borders
  and whatnot
- instead, style its CSS nodes using a hardcoded CSS provider and simply
  "know" the size of the slider in draw(), did this in code instead of
  the theme CSS because we do pretty evil things here and we don't want
  theme authors to think they can change much about it