[Feature] add parameters to calibrate display for auto-brightness

[luclu]

trafficstars

Rationale As the auto-brightness feature uses the internal brightness sensor (e.g. on Macbook) the external display might not match the same brightness curve. This might result

Proposal

1. add n parameters representing a polynomial function of n-th grade (e.g. y = a*x**2 + b*x + c), i.e.
   • n=1: only offset
   • n=2: offset + slope
2. implement a graphical interface instead of input field for the parameters