When restarting (X compositor only, obviously), we want to keep
the same window focused. There is code that tries to do this by
calling XGetInputFocus() but the previously focused window will
almost certainly not still be focused by the time we get to the
point where we call XGetInputFocus(), and in fact, probably was
no longer correct after the previous window manager exited, so
the net result is that we tend to focus no window on restart.

A better approach is to leave the _NET_ACTIVE_WINDOW property
set on the root window during exit, and if we find it set when
starting, use that to initialize focus.