X.A.Prefix: Use EZConfig parser for usePrefixKey
X.A.Prefix: Use EZConfig parser for usePrefixKey

Utilise EZConfig's Emacs style syntax for entering the prefix key.

The syntax has been well-established for use in configuration files;
especially in this case it can be a bit awkward having to specify a
function that takes a config just for a single keybinding.

xmonad/xmonad-contrib,xmonad/xmonad-contrib