timeline-layer: Fix track-added Callback parametters
timeline-layer: Fix track-added Callback parametters

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