base-xml-formatter: s/ducation/duration/
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.
clip: Handle child priority offsets when setting priority
track: Make sure new that gaps are filled before removing the old ones

Currently we can end up having gaps in track as the first step of the
gap filling method removes currently set gaps.
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
clip: Avoid list corruption when grouping objects

We are currently iterating over a list that is modified in the same
method, we have to get a copy of the list, and iterate over the copy.
clip: Emit the notify::layer signal only when actually needed

That means:
  - only when we do change layer
  - At the end of moving between two layers
group: Implement a GESGroup class