timeline: Use g_sequence_sort_changed when appropriate
timeline: Use g_sequence_sort_changed when appropriate

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