Make the server handle arbitrary pipelines

Make the server handle arbitrary pipelines

Make GstMediaFactory an object that can instantiate GstMediaBin objects.
The GstMediaBin object has a handle to a bin with elements and to a list of
GstMediaStream objects that this bin produces.

Add GstMediaMapper that can map url mountpoints to GstMediaFactory objects along
with methods to register and remove those mappings.

Add methods and a property to GstRTSPServer to manage the GstMediaMapper object
used by the server instance.

Modify the example application so that it shows how to create custom pipelines
attached to a specific mount point.

Various misc cleanps.