Don't directly expose the state change methods. Only expose an activate
method for nodes that indicates if the node should be scheduled or not.
Let the link automatically negotiate two active nodes.
Make some defines for properties.
Use link property to make passive links. Passive links don't activate
the nodes.
Make sure the mainloop quits when quit is called before run.
Add signal handlers for daemon to shut down
Add destroy listeners for modules and do cleanup
Fix some leaks
Simplify port registration in new nodes
Hide some more data structures
Let the node implementation take care of the reuse_buffer signal because
then we can get to the graph nodes to find the destination port.
Destroy modules in core cleanup. Modules should undo everything they
have done.
Activate the link after we negotiated format and buffers.
