Add function to swap hands / mirror layout

[t0ysw4r]

Hi,

I'm left handed... Moreover the qwerty layout is left heavy.

It would be nice to select a swap hands button (for behaviour and appearence). This would mirror the keys in the simulator, so that right hand heavy layout can easily be tested but with a left heavy hand.

[Beethoven-n]

people have suggested that i do mirror nerps after i said that i preferred vowels on the left hand! it'd be nice to be able to do this

[Beethoven-n]

there's now an issue about it!
#5573