Add support for nested timelines

Basically we need a way to have timelines inside timeline.

The implementation idea is to have GStreamer elements that are able to playback "serialized timelines" a.k.a gesdemux with a typefind function and simply reuse GESUriClip to play those. Then we need to be able to serialize projects with nested timelines inside xges files, and we want to be able to have several timelines serialized in one single xges file so we need a way to determine what timeline to load from the URI basically.

With !33 (merged) we are now able to load serialized timeline, even inside GES with ges-launch-1.0 +clip file://some/file.xges which reveals issue in nlecomposition but it is all fixable.