timeline-layer: Implement the Extractable type
timeline-layer: Implement the Extractable type

We can imagine user implemts more Layer type, it could be usefull for formatters
to instanciate through a GESMaterial

pitivi/gst-editing-services,cfoch/gst-editing-services,cfoch/gst-editing-services,cfoch/gst-editing-services,pitivi/gst-editing-services,cfoch/ges,cfoch/ges,pitivi/gst-editing-services,pitivi/gst-editing-services,cfoch/gst-editing-services,cfoch/ges,cfoch/ges