timeline-standard-transition: Override the GESExtractable implementation
timeline-standard-transition: Override the GESExtractable implementation

Standard transition material have the vtype property as ID, it has the particularity
that the ID can be changed at runtime

	+ Implement tests to make sure it behaves properly

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