timeline: create_transitions_on_layer *before* actually commiting

Everything need to be in place before commiting, otherwize it makes no
sense at all.
track: Make it possible to disable the mixing feature

API:
  ges_track_set_mixing
  ges_track_get_mixing
framepositionner: fix messup with propname enum.
track-element: No need to log when prio == MIN_GNL_PRIO.
test-utils: Add test file generation code.
tests: Add integration tests

Those are test with real media files, they are run separetely from other
unit tests using the make check-integration command (can be done from
the toplevel directory)
tests: Simplifie integrations tests using macros all around