Add options for button placements.

[DavidSM64]

I love the app. But one very minor annoyance I have with it is the layout of the hex digits. I'd like to be able to move them around so that the 0-9 are grouped together like the decimal tab and the A-F are to the left side. Kinda like how it is done on the Windows calculator. I know this is not a trivial addition, but more customization would be a great quality of life improvement.