[xmonad] Prompt.RunOrRaise

[Don Stewart]

mail:
> This patch implements a prompt which runs an application if it's not 
> running, raises the existing one if it is running, or opens a file using 
> xdg-open (which selects a preferred application) if you select something 
> that isn't executable. The implementation is a little hairy, and code 
> cleanup would be very welcome. Let me know what you think.

Applied. Great to see new contributors.

Thanks.

 -- Don