ges: Enhance xges format versioning

ges: Enhance xges format versioning

Summary:
Handle the fact that some new features can be added and that means
generated files will not be fully understandable by older versions of
the formatter.

Make sure that we set the format version to 0.2 when we serialize the
GstEncodingProfile.enabled property.

Add some tests around that.

+ Fix a minor bug in the test-utils
+ Add a meta on the projects to tell in what format version a project
  has been serialized/parsed back

API:
  GES_META_FORMAT_VERSION

Depends on D178

Reviewers: Mathieu_Du

Differential Revision: http://phabricator.freedesktop.org/D184