clip: Rework the splitting method

  + Avoid setting clip duration of our parent ourself

Now each and every TrackElement inside a clip have the same
start/inpoint/duration
tests: Check splitting a clip with several TrackElement
project: Accept NULL as a valid value for @id in _create_asset
ges: Fix compilation with clang
uri-clip-asset: Add "_class_" to a class method

API CHANGE:
	- ges_uri_clip_asset_set_timeout
	+ ges_uri_clip_asset_class_set_timeout
xml-formatter: Use G_GUINT64_FORMAT where needed
Fix compilation
Automatic update of common submodule

From 3cb3d3c to 5edcd85
formatter: Pass a dummy instance of formatter to virtual method

Instead of passing the class itself
trackelement: asynchronously add bindings if the track-element is not in a track yet.

Also fix annotations.
layer: Remove the "rate" property of ges_timeline_layer_add_asset

API:
 - ges_timeline_layer_add_asset (layer, asset,  start,  inpoint, duration, rate, track_types);
 + ges_timeline_layer_add_asset (layer, asset,  start,  inpoint, duration, track_types);
Rename GESTimelineLayer to GESLayer
replace query_caps with get_current_caps. don't check for unused gnlobject.
timeline: add auto-transition

API:
 ges_timeline_get_auto_transition
 ges_timeline_set_auto_transition
 GESTimeline::auto-transition
tests: add timeline auto-transition test
tests: add project auto-transition test