They were not serialized until now.
That implies several changes:
* Override GESTimelineElement [start, inpoint, duration] properties in
GESGroup to ensure that those properties are not serialized as they
should not be.
* Rename GESBaseXmlContainer->clips field to
GESBaseXmlContainer->containers as the hashtable now contains Groups
https://bugzilla.gnome.org/show_bug.cgi?id=709148
And fix all the tests as we need to wait for the project to be loaded
to check the reference count of the timeline (as we keep a ref on the
timeline in project to later emit "loaded" on idle).
Currently we can end up overflowing the start of others child of our
parent, avoid that making sure we can set our start to what was
requested by the user before actually doing it
+ Add a test
