The listeners are generally useful in spa, move it there and rename
to hooks. Implement loop hooks with it.
Fix some cleanup issues in stream and remote
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.