ges-xml-formatter: use g_ascii_dtostr() instead of messing with setlocale()

Libraries shouldn't use setlocale().
container: Do not forget to initialize the timeline before using it
ges-source: Move common elements handling to the base class

  + And port all the subclasses
source: Make a ges_source_create_topbin internal helper method
testsource: Handle child properties as child properties

Makes $make check pass.
Standardizes property handling.
GES: Add GESVideoSource and GESAudioSource base classes

  + Update documentation.
  + Implements subclasses audio-uri-source and video-uri-source
ges-track: Add the notion of resriction caps to GESTrack

This way we can let the user determine what he want to come out of the
track.

API:
  - ges_track_set_caps (The track caps are now construct only)
  + ges_track_set_restriction_caps
  + ges_track_get_restriction_caps
  + GESTrack.props.restriction_caps
gstframepositionner: Install width and height properties.

  + And manage them properly.
videotransition: No need to hard set width and height anymore.
test-utils: Adds a utility function to quickly check the timeline.
tests: timelineedition: Add a test_scaling.

It will check that the clip updates its size correctly.
timeline-element: Add a set_parent vmethod

API:
        GESTimelineElment->set_parent vmethod
videosource: Make sure to update z-order when layer priority changes

Conflicts:
	ges/ges-video-source.c
gstframepositionner: correctly tag metadata.

We do not use GST_VIDEO_META_TAG_STR as it would mean depending on
GstVideo which is not the case right now
ges: Make GESAudioSource and GESVideoSource abstract
timeline: Make ripple start be trimming

This is a more natural behaviour as a user instead of doing nothing at
all.
ges: Handle trimming in groups

This was broken, clips where moving all around, make it behave properly.
timeline: Avoid setting duration > max_duration when rippling

We should use the trimming method to set duration to make sure to avoid
going over the max duration.

Also avoid computing when setting duration to the same old value.
timeline: Make sure we do not move object when only trimming

We were missing a few checks so that we do not move objects when their
duration is equal to the max duration, or 0
tests: timelineedition: Minor cleanups