GESTimelineObject: Update TrackObject priorities handling
GESTimelineObject: Update TrackObject priorities handling

make use of the new TrackObject getters

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