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).
* max-duration is the total length of the File.
* supported-formats is the various track types this filesource can produce
trackobjects for. This should maybe be moved to parent classes in the
future
Step 1 of GstDiscoverer integration