ges: Move ges_clip_edit to GESContainer

This exact same method will be needed in GESGroup, so we should have the method
in the common parent class.

API:
    - ges_clip_edit
    + ges_container_edit
    + GESContainer->edit vmethod
container: ges_container_ungroup return a transfer full list
container: Let subclasses decided when height change should be computed

API:
   - GESContainer.compute_height vmethod
   + _ges_container_set_height
ges: Avoid leaking the timeline when grouping containers
container: "Implement" the set_priority vmethod

This way we will just accept any value setted
container: Do not allow adding an element to a container if it already has a parent

This should never happen, an element can have 1 and only 1 parent.
group: Implement a GESGroup class