#271 Consider using xdg standard for config files

[William Kendrick]

@thomasklausner, who maintains the NetBSD pkgsrc port of Tux Paint, suggests:

I notice that tuxpaint is using .tuxpaint - can you please consider
switching to the xdg standard directories? See
https://specifications.freedesktop.org/basedir-spec/latest/ but
basically it means that configuration is below .config, e.g. in
.config/tuxpaint and the home directory is cleaner.

(See also https://sourceforge.net/p/tuxpaint/feature-requests/103/)