Two-layer, inter-Instance communication

[vonglan]

Low priority.
An idea from the discussion of #109 .

It takes the "multiple ReaLearn instances" concept one step further:
Decouple controllers and VSTs, by allowing two (not more) layers of ReaLearn instances:

• Layer One would hold one ReaLearn Instance for each Controller (like described above, using only the Controller Mapping compartment).
• Layer Two would hold one Instance for each VST in that track (using only the Main Mapping compartment).
• They would communicate via Virtual targets/sources in exactly the same way that Controller Mapping and Main Mapping can currently communicate inside one Instance.

You could also describe this concept as: allowing the user to separate Controller Mapping and Main Mapping into different ReaLearn instances, and arranging for communication between them as if they were in one instance.

In some cases, this would reduce the number of required ReaLearn instances.
Another advantage of this concept would be that I would not have to load/copy the preset for the Main Mapping to the instrument VST redundantly into the ReaLearn instances.