This will give us margin for API expansion without breaking ABI.
The ABI restriction will only come in place once we do the first
official release (i.e. 0.x.0).
If a GESTimelineFileSource is added to a layer and:
* It doesn't have specified supported formats
* OR it doesn't have a specified maximum duration
* OR it doesn't have a specifed duration
Then we asynchronously send it to the GstDiscoverer.
If this happens, the state change of the timeline from READY to
PAUSED will happen asynchronously and be completed when everything
has been properly discovered.
Part 2 of GstDiscoverer integration
