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 integration tests using macros all around
ges-uri-clip: Add the possibility to specify an assets directory

through the GES_TESTING_ASSETS_DIRECTORY environment variable.
integration: add SeekInfo and get_position callback for seek tests

A Seekinfo structure consists of 2 fields:
- position: the position to seek to
- seeking_position: the position to perform the seek from

Seeks can be appended to a global list e.g. from code:
seeks = g_list_append (seeks, new_seek_info (0.2 * GST_SECOND, 0.6 * GST_SECOND));
seeks = g_list_append (seeks, new_seek_info (1.0 * GST_SECOND, 1.2 * GST_SECOND));
seeks = g_list_append (seeks, new_seek_info (1.5 * GST_SECOND, 1.8 * GST_SECOND));

The get_position callback checks the current position and attempts to perform
the corresponding seek with gst_element_seek_simple
integration: some fixes for seek tests

- Use g_list_remove_link so that ordering of seeks is not mandatory
- use g_slice allocator for SeekInfo structs
- Fix leak in freeing seek list
- Check for NULL seeks at end of test, otherwise fail and free failed seeks
tests: Remove prints

And use GST_DEBUG and friends instead
tests: integration: Remove the effect in test_seeking

It complexifies the test but this is not what we actually want in
that test
tests: integration: Add audio/video only seeking tests
WIP
tests: integration: Refactor and makew easier to add encoding profiles