timeline: Track TrackObject-s by layer
timeline: Track TrackObject-s by layer

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