#6 Add support for routing

[Ken Ellinwood]

Fluidsynth has routing commands which can be used to
create keyboard splits and layering. Add support for
this in the GUI.

Idea: Implement a tree widget with one sub-item per
channel. Under each channel is the list of layers. If
nothing has been done yet, a default layer is shown.
Layers may be "split" and layers may be added to other
(groups of) layers.