uri-source: Expose the volume property.

	+ Add a create_props_hashtable implementation
	+ If TRACK_TYPE_AUDIO, put the volume properties in the hashtable.
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.