Make OakPasteboardSelector accessible

Issues remaining:

* VoiceOver for some reason cannot track keyboard focus
  (i.e. moving down the list with just down arrow does not move
  VoiceOver cursor). User has to move the VO cursor instead using VO-down
  (and keyboard cursor follows in this case)
* the pasteboard window isn't focused automatically for VoiceOver, user
  has to switch manually to it using VO-F2-F2

Released to public domain by my employer, BRAILCOM,o.p.s.