Use XDG base directories to store configs and data

[amhndu]

nstdir now points to $XDG_DATA_HOME/nestopia/ and config files are stored
in $XDG_CONFIG_HOME/nestopia/ to conform to XDG base directory
specification.

This is a preliminary pull request, I've simply changed the respective hardcoded paths.
It does not consider compatibility with previous configuration, or trying it move it over etc.

Motivation:
A majority of desktop applications and environments have now switched to using XDG paths,
and this leads to a more organized file structure and uncluttered home directory, among other things.
Spec: https://specifications.freedesktop.org/basedir-spec/basedir-spec-latest.html

[rdanbrook]

Sorry, this isn't something I'm interested in changing. Many programs have not moved over to XDG paths, and this is one of them. I plan to keep it that way for the foreseeable future.