timeline: Make use of our own knowledge of the timeline duration
timeline: Make use of our own knowledge of the timeline duration

Do not use each Track durations as it end going in loop as we have the Tracks
that need to know about timeline's duration to create or not gaps in the end and
then the timeline references on Tracks duration for its duration. We have this
information locally so just make proper use of it.

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