Same or derivate project can be loaded in more than one room. This will cause more than one port
with given uuid to be added to the jack graph. This changeset uses vgraph-port_uuid pair when searching
for port that was added at earlier project load stage. For this to work, when ports are created and added
to the jack graph, the vgraph handle is stored in the port object.
There are three types of ports depeneding on references to them:
* ports that are referenced from jack and studio graphs
* ports that are referenced from jack and room graphs
* studio-room link ports that are referenced from studio and room graphs
The old approach was to destroy ports when removing references from jack graph.
Studio-room link ports were destroyed when removing the room.
However this caused double free when removing app ports that appear in jack and
room graphs.
The introduced implementation uses reference counting of graph references to ports.
Temporary references (like the one returned by port constructor and graph_get_port())
are not tracked.
Nedko Arnaudov