docs: Misc documentation fixes
timeline: create_transitions_on_layer *before* actually commiting.
ges-track-element: call virtual method duration_changed when ...

duration has changed !
track-element: Remove duration_changed virtual method.

We use notifies for the properties.
	+ Use notifies in audio-transition and video-transition
track: adds set_mixing and get_mixing.
Test-utils: Add test file generation code.
tests: Add rendering tests.
Rendering tests: WIP
tests:render: Add MOV tests, as they are a classic camera format (canon 5D)
Rename render to integration.

	+ This is the real scope of that branch
	+ Adds a check-integration rule.
tests/integration: do playback and render tests.
integration tests: add a forever test.
tests/integration: port to commit based API.

	+ Only set_render_settings on the pipeline when necessary.
tests/integration: add the stem of a (already failing) new test.
framepositionner: fix messup with propname enum.
track-element: No need to log when prio == MIN_GNL_PRIO.
integration: add the stem of an image test.
integration: adds a video layer to the image test, playback doesn't start.
tests: integration: Misc cleanup
API: container: Add a recursive argument to _get_children.