timeline: Add all assets of the clip added to the timeline to the project
Revert "ges: Always create a project to back a timeline"

This reverts commit 59d83f1a93055391097e7c1fe34f5a39eb8ec625.

Conflicts:
	tests/check/ges/backgroundsource.c
	tests/check/ges/effects.c
	tests/check/ges/overlays.c
	tests/check/ges/simplelayer.c
	tests/check/ges/text_properties.c
	tests/check/ges/titles.c
ges: Reimplement 'always create a project to back a timeline'

Keeping it simple, and making sure everything is synchronous
ges: multifilesrc support

* Add GESMultiFileSource class
* Add multifilesrc example
* Support multifile:// urls in uri asset
free and unref. copyright
multifilesrc: add start/stop index modification
multifilesrc: move uri prefix to define
multifilesrc: add doc and fix gir warnings