Implement GESAudioTrack and GESVideoTrack, subclasses of GESTrack
timeline-element: Make it possible to reset the timeline property to NULL

	+ Add some debug symbol
layer: Simplify a bit how we handle priorities
layer: Fix some mix up in variable names
tests:effects: Make use of normal layers, and enhance tests
Make space in the GESTracks to be able to add mixing elements later

And update the tests
track-element: Set a reference to the GESTrackElement on the GnlObjects using qdata
track: Implement infrastructure for mixing
Misc documentation fixes
meta-container: Add a VOLUME default meta to layers
Implement a GESSmartAdder bin element to be used as mixing element

..in audio tracks