Allow entering offset for Slow control

[Patschke]

Is your feature request related to a problem? Please describe.
We'd like to lock a Dye laser using linien. The resonator features two Piezo mirrors, one with a high bandwith (that can only scan a few hundred MHz) and one with a high scan range (that is slower but allows to tune across many GHz). Obviously we'd like to control both via Linien.
To tune the laser to our lockpoint, the slow output needs to be set to the correct region (the Redpitaya directly feeds the Piezo driver). I did not see an option within the Linien GUI to change the output of AOUT 0/slow control manually.

Describe the solution you'd like
An input box to control the offset for AOUT 0 (like it already exists for AOUT 1-3).

Describe alternatives you've considered
We can use a voltage adder and combine AOUT 0 with one of the other AOUTs.