feat(developer): better control over width and layout of touch layout (centering, no auto-key-sizing)

Feedback from FV team (@caforbes, @HopsAndHops, @xyzhelen):

> Don’t have the right-most key get bigger to fill up the remaining space if no spacer is put
on the side - the solution is sometimes easy, but trying to center the keyboards is
frustrating.
> * Dreaming big: Some other easy way to center the keys on the touch layout??