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.
