uri-source: when creating the audio element, set the volume to the layr volume when necessary.
uri-source: make the volume controllable.

	+ 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.