ges-uri-source: Refactoring work.

	+ Categorize functions (Callbacks, vmethods)
	+ make more generic functions for the creation of the bin.
ges-smart-adder: ges-meta-container.h is not needed anymore.
ges-layer.c: notify priority changes.
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.
ges-smart-mixer / ges-uri-source: add a framepositionner element.

It adds metadata on the buffers and the mixer parses them.
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.