UI: remove unused code
UI: events: detect activation and cancellation key events
UI: item: allow generic activation of items
UI: window: allow forced drawing of a child item
UI: window: activate focus widget on Space or Return
UI: buttons: implement generic activation for buttons
UI: allow scrolling a container to show a specific descendant
UI: renamed activate_item(), ensure activated items are visible
Merge branch 'scroll-and-activate'

* scroll-and-activate:
  UI: renamed activate_item(), ensure activated items are visible
  UI: allow scrolling a container to show a specific descendant
  UI: buttons: implement generic activation for buttons
  UI: window: activate focus widget on Space or Return
  UI: window: allow forced drawing of a child item
  UI: item: allow generic activation of items
  UI: events: detect activation and cancellation key events
  UI: remove unused code