container: Let full control of children priorities to subclasses

For that we make the children_control_mode a protected filed, directly usable by
subclasses, removing the method to set it.

And we let the subclass set and get the priority offsets to the container class.
container: Remove the get_priority_range vmethod

We now let full control to subclasses so we do not need it anymore.
clip: Add an internal method to easily get the priority of the layer the clip is in
group: Implement a GESGroup class