GESTimelineStandardTransition: keep track of TrackVideoTransition
GESTimelineStandardTransition: keep track of TrackVideoTransition

Use the new track_object_added/release vfunc to get cleaner code

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