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
timeline: create_transitions_on_layer *before* actually commiting.
ges-track-element: call virtual method duration_changed when ...

duration has changed !
track-element: Remove duration_changed virtual method.

We use notifies for the properties.
	+ Use notifies in audio-transition and video-transition