tests: More safely check if objects where destroyed

Check if an object rthat has already been freed has been destroyed is not safe.
Add a helper function that uses weak reference to check that objects that are expected
to be destroyed when unrefing an object are actually destroyed.
timeline-element: Add a method to get the topelevel parent of an element

API:
	ges_timeline_element_get_toplevel_parent
group: Implement a GESGroup class