Feature: Fade brightness when set/changed via command line

**Why**
I have an external light sensor which I use to send new brightness values to Twinkle Tray via the command line: **--All --Set=**_{brightness}_

**Wish**
Sometimes there is a 5 points brighness difference between the current value and the new value, it would be nice if Twinkle Tray steps towards the new value with 1 increments. Delayed with the _brightness update rate_ in monitor settings.




