UI: allow "-" in command names
UI: introduce Adjustment::move() to implement forward/backward step/page moves
UI: use Adjustment::move to implement SliderArea::move
UI: support left/down/right/up scrolling in ScrollPortImpl
Merge branch 'area-scrolling'

* area-scrolling:
  UI: support left/down/right/up scrolling in ScrollPortImpl
  UI: use Adjustment::move to implement SliderArea::move
  UI: introduce Adjustment::move() to implement forward/backward step/page moves
  UI: allow "-" in command names