xStream: ability to stack models / specify track routings

[bjorn-nesby]

Stacking: allow models to pass input on / or receive from one another.
Routing: allow models to be linked to specific tracks (reading and/or writing)

Will require a lot of fundamental changes, such as:

• Abstracting the stream process (stack is basically multiple streaming processes)
• Persisting settings (allow saving configurations within the Renoise project)
• A GUI for working with / switching between multiple models