Add hotkeys to switch between screen emphasis and screen even sizing

[lucianposton]

Just throwing this up on github in case others were looking for a way to add a hotkey to switch screen emphasis between bottom/top or even/emphasis until melonDS's QT ui is finished.

[Arisotura]

I like this idea; might also have one for cycling through screen rotation

however, we switched to a Qt UI, which is why I didn't merge this; if you port this to the Qt UI, I will merge it