Use XDG spec dir to store config/user data

[Mazek]

For Linux build, it would be nice if XnViewMP complies this specification: http://standards.freedesktop.org/basedi ... atest.html

Instead of lumping everything into a dir in the root of user's home dir, segregate configs, data and cache into ~/.config and ~/.local/share and ~/.cache according to the guideline in the basedir spec link. You would have to check whether corresponding $XDG_* env vars are set and only default to those aforementioned dirs if they are not

It's a good thing to have centralised and organised data storage and also not cluttering user's home directory