ges-smart-mixer: first code dump, mainly copy paste from ges-smart-adder.
video-track: "implement" get_mixing_element.
tests: Add a audio video mixing test, which kind of works.

	+ To see it kind of work, you'll need the patch attached there : https://bugzilla.gnome.org/show_bug.cgi?id=701237
ges-smart-mixer / ges-uri-source: add a framepositionner element which

... adds metadata on the video buffers and parse them in the smart-mixer.
	+ This is done because we want to keep positionning properties
	+ and set them on the dynamic mixer pad.

Conflicts:
	ges/Makefile.am
ges-uri-asset.c: Fix ges_uri_asset_request_sync annotations.
ges-smart-mixer: use a black background.