[FEAT] Nudge viewport up/down/left/right using hotkeys

[WillLem]

As of the next update (2.9 or 2.8.5), we now have hotkeys for nudging the viewport up/down/left/right by a variable amount (set in the hotkey config; the default is 160px).

This is particulary relevant when using SLX with the minimap hidden, but will also make playing with a gamepad easier to manage. The feature is one I suggested here for NeoLemmix a while ago, and I've finally found the correct formula to get it working.

Implemented in commits b2ef225 and 73c84b3. The action itself comes down to a single line of code; it's always the hotkey management that's the most cumbersome part.