Rework the node and port API so that other implementations can be used
than the spa_node. The plan is to morph this into the stream and
context API.
Rework the graph API a little so that init + add is separated.
Make real spa_graph nodes and ports and schedule those. This makes
it possible to add explicit tee and mixers in the real graph.
Rework the way we add and remove ports and nodes from the graph.
Remove confusing pw_port_link and merge core with pw_link_new()
Move scheduling in separate files, add some more graph-schedulers.
Trigger initialized after we created the node global so that we
can get its id.
Add the server side node_id for the stream in the stream struct.
Change the transport function to pass the node_id to the client.
Small cleanups
