timed-revealer: Inherit from GtkEventBox
    
    As suggested by Christophe, inheriting from GtkEventBox instead of
    having one instance of it as a member can help us to get rid of
    virt_viewer_timed_revealer_get_overlay_widget().
    
    Signed-off-by: Fabiano Fidêncio <fidencio@redhat.com>
    Acked-by: Christophe Fergeau <cfergeau@redhat.com>