TimelineObject: adapt the add_track_object so we can add effects to it.
TimelineObject: adapt the add_track_object so we can add effects to it.

We keep the list of contained TrackObject-s order by priority

pitivi/gst-editing-services,freesteph/gst-editing-services,MathieuDuponchelle/PitiviGes,cfoch/ges,ahmedammar/platform_external_gst_editing_services,luisbg/gst-editing-services,MathieuDuponchelle/PitiviGes,cfoch/gst-editing-services,MathieuDuponchelle/PitiviGes,pitivi/gst-editing-services,cfoch/gst-editing-services,volodymyrrudyi/gst-editing-services,pitivi/gst-editing-services,freesteph/gst-editing-services,cfoch/gst-editing-services,ahmedammar/platform_external_gst_editing_services,cfoch/ges,volodymyrrudyi/gst-editing-services,volodymyrrudyi/gst-editing-services,freesteph/gst-editing-services,MathieuDuponchelle/PitiviGes,luisbg/gst-editing-services,cfoch/gst-editing-services,cfoch/ges,cfoch/ges,pitivi/gst-editing-services