Make explicit links between elements that are used to activate the
next element in the graph.
Make subgraphs a special regular node. Make a link from the
subgraph children to the parent so that the subgraph completes when
all the children completed.
Implement a single process function in plugins
Remove many messages in the client node
Move the activation state to the graph
Replace an old scheduler with a new one that works with the new
activation states.
Remove the DISABLED port flags, we explicitly add and remove to
make ports enabled/disabled.
Find first compatible port for link
Remove the node based scheduler and use the spa one.
Make enum_params and set_param to configure properties, format
and other parameters. This allows us to remove some duplicate
code and make the properties and parameters much more extensible.
Use the object id to mark the id of the parameter.
Remove the spa_format and spa_props.
We can now make the client-node easier by merging the various
format methods into the params.
Make the stream API more powerful now that we can pass params
around.
