* clutter/clutter-container.[ch]: Add checks to the Container
	interface invocation methods, to avoid crashing or corrupting
	the stack when an actor does not implement every virtual
	function of the Container interface vtable. GObject allows this
	to happen so we must handle the case gracefully. This also means
	that we can classify some virtual function as mandatory (as is
	the case for ::add, ::remove and ::foreach) and some other
	optional.