PODxt: signal routing & monitor level dialog

[arteme]

Line6 drivers have an "Inputs and Recording" section where the end-user can control signal routing & monitor level. Would be a good idea to implement the same, since in Linux especially, there's no way to select different signal routes.

Seems like the signal routing is:

f0 00 01 0c 03 57 05 00 00 00 00 f7 -- request
f0 00 01 0c 03 56 05 xx xx xx xx f7 -- reply

where the 12-bit xx xx xx xx value is:

• 00 -- processed guitar output over USB
• 01 -- clean guitar output over USB
• 02 -- send clean guitar over USB, re-amp audio received from USB
• 03 -- re-amo audio received from USB

And the monitor level is:

f0 00 01 0c 03 57 04 00 00 00 00 f7 -- request
f0 00 01 0c 03 56 04 xx xx xx xx f7 -- reply

where the 12-bit xx xx xx xx value is 0x0000 - 0x8000