GitLab

Do not emit the signal from outside of the class
Remove GUIMedia class

This introduces 3 interfaces:
- IBackend: Provides the very basic functionnalies associated to a media
  backend (log hook & creating sources)
- ISource: Provides functionnalities associated to a media source. Basic
  metadata handling & spawning renderers to use this source
- IMemorySource: Describe a source which is described programmaticaly
  and fed from memory
- ISourceRenderer: Allows one to render and configure render options.

On paper, this is feature complete with regards to the old
implementation.
In practice, it is not, and pretty much all media related
fonctionnalities are broken, and will require further refactoring to
become functionnal again.

Instead put the clip workflow in error state

ClipWorkflow won't stop itself but will signal TrackWorkflow instead.