Channel Config: Implement TRIM for AUX Input 1-6

[schulz-alexander]

If the channel has AUX 1-6 as input, use the GAIN Knob as TRIM

60dB Gain == 0dB Trim ("unpadded" signal)
-18dB Gain == ??dB Trim

TODO:

• change GUI if the Channel Input is an AUX-Input