a) remove focus_hilite, it is not needed and complicated things

a) remove focus_hilite, it is not needed and complicated things
b) set focus_client to null when nothing is actually focused, but still allow
   focus to go to black holes
c) allow the focus action to be performed without a client, this will focus
   the openbox instance (i.e. the screen in multihead setups)

big thanks to syscrash for the ideas on how to go about this