compositor: Add an API to focus the stage X window

gnome-shell has traditionally just called XSetInputFocus when wanting to
set the input focus to the stage window, but this might cause strange,
hard-to-reproduce bugs because of an interference with mutter's focus
prediction. Add API to allow gnome-shell to focus the stage window that
also updates mutter's internal focus prediction state.

https://bugzilla.gnome.org/show_bug.cgi?id=700735