refactored deck editor UI key shortcuts

The global shortcuts were removed in favor of key events
which are only triggered when a widget has focus. This is
necessary to allow different widgets to react differently
to the 'enter' key. The current key configuration is:
search edit focused:
  * left/right: decrease/increase card count for selected database card
  * enter: add selected card to deck
  * ctrl-enter: add selected card to deck sideboard
  * ctrl-left/right: decrease/increase card count in sideboard for selected card

database focused: the same as with search edit.

deckview focused:
  * left/right: decrease/increase card count for selected deckview card
  * enter: increase count for selected deckview card
  * delete/backspace: delete selected card from deck