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.
Replace the AsyncDone event with an explicit done callback in the node
to signal completion of an async operation.
Pass read and write fd together with the transport
Make a default logger and mapper in a .h file to be used by examples
Remove logger and mapper from libs
Make method to set the default mapper for the debug methods
Improve event and command init so that it can be used more easily
as compound literals.
Improve volume
Make it possible to use graph scheduler in test-mixer
