Use XDG base directories by default under Linux

[superuser-does]

It would be nice if was to follow the XDG Base Directory Specification. It is intended for graphical/desktop software, which includes games.

Benefits for players

• They will have one less hidden directory in their $HOME folder.
• Backup solutions for configuration and data can automatically find files, without the player specifically having to add ~/.ja2 within their backup configuration.
• It is preferred for distribution-independent packaging methods (e.g. see Flatpak documentation).
  However, I cannot say how exactly this is used.

Implementation

The logic used by Cmus* is an effective model to follow. Please note that this program had a specific variable for its directory location
At first instance, ~/.ja2 should be used if it already exists, to enable backward compatibility. However, new players should have XDG folders as their default.

To follow the specification closely and in line with other games which implement it, Stracciatella should:

• Store configuration in XDG_CONFIG_HOME or $HOME/.local/share if unset
• Store saves and mods in XDG_DATA_HOME or $HOME/.config if unset

It is also a chance to go further if people want, and change the default folder name from .ja2 to Jagged Alliance 2, which is more commonly accepted under Linux at this point.

History

I am filing this enhancement following #1864 being implemented on Windows. A desire to follow this specification was also mentioned as early as 2016 in #164 (comment).