Major churn for multi-headed support (Xrandr and plain multi-screen).

Major churn for multi-headed support (Xrandr and plain multi-screen).
- workspaces now per-screen (X does not let you migrate windows)
- a screen has 1 or more regions (now mapped by Xrandr, later configurable)
- current_ws is dead, long live cur_focus
	- only one window has real focus, but workspaces keep track ws->focus
	- lots of places (especially event handlers) use cur_focus but should
	  identify the correct source of the event instead
- festival of pointer management to keep track of all this.