compositor: Don't bother selecting for events on the COW

We already select all the events we need on the stage window instead.
screen: Don't select for key and motion events on the root window

These aren't necessary, and we'll never see these events anyway, since
we don't process input events on the frontend connection.
screen: Select for events on the guard window on the backend connection

Otherwise, we'll never see them. This fixes the background menu not
working in gnome-shell.