Rework pad-added algorithm.

Make all the code more generic allowing to create more than one branch
starting from the same source.
Add pipeline with appsink.
Use the recursive branch building also for audio.
Fix missing % character.
Add utils module.
Reshape volume input gui.
Add AudioInput class.
Add element message management by pipeline.
Factorize code.
Add class V4L2StreamStudioSource to manage v4l2 source.
Change signature of stream-added signal.

Now pass also the stream id so upstream we can differentiate behaviour.
Add management of level and volume.
Fix glade interface.
Add StreamStudioMonitorInput.
Update script to latest changes.