Implement initial support for XDG [1]

I only used `Config` as I do not know what everything that calls PfaEditDir actually is, so I did not try to decide where each file belongs.  I replaced the HomeDir function and avoided using existing GFile functions, as they use `copy()` which leaks.
 
Also, the files will not be created if `XDG_CONFIG_HOME` or `~/.config` does not exist.  Something in `ffensuredir` would fix that.  Finally, because I have replaced getPfaEditDir completely, consideration for the `VMS` flag and renaming of `~/.PfaEdit` is gone.

#847