Some more subtleties for the focus handling code. In particular:

Some more subtleties for the focus handling code.  In particular:
- A global last_focus_event helps us to determine whether another process
  is playing with focus without telling us.
- merge focusin and focusout into a single focusevent, handle focus change
  events triggered by synergy.
- Fix focus issues when entering a fullstack window from another region, by
  checking to see which ws the focus is coming from before ignoring the
  event.