Handle crossing events with subwindows unknown to gdk

Handle crossing events with subwindows unknown to gdk

If we get crossing events with subwindow unexpectedly being NULL
that means there is a native subwindow that gdk doesn't know about.
We track these and forward them, with the correct virtual window
events inbetween.

This is important to get right, as metacity uses gdk for the frame
windows, but gdk doesn't know about the client windows reparented
into the frame.