container: Make initiate_move a read only protected member

It is a interesting information for subclasses.
Conflicts:
	ges/ges-container.c
ges: Move GESTimelineElemt vmthod implementation from container to clip

This is where they belong to has they are specific to that
implementation of the baseclass
container: Fix the way we check prirority of subclasses when grouping objects

The resulting list was from lower to higher, we need the contrary
track: Update gaps only when commiting

We were still updating them at useless moments, do it only when absolutely needed.
internal: Add a macro to make it easier to get the end of a TimelineElement
clip: Never try to set the start after the end of an element when triming
timeline: Create transitions only when commiting
docs: Misc documentation fixes
container: Update offsets in GESTimelineElement vmethod implementations

So subclasses just have to link up to resync offsets
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
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.