We just need to make sure they are always serialized in the right
order (which is the case) and de serializing them will lead to the
right behaviour.

We should not serialize the priority as the priority of the source
itself depends on the action having been done on the parent clip,
and we do not serialize the source priorities (and should not, GES
should just do the right thing).

Differential Revision: https://phabricator.freedesktop.org/D491