Respect XDG Base Directory Specification

[Neurognostic]

Please respect the XDG Base Directory Specification.

Instead of placing all files in the non-standardized ~/.weasis directory comply with the standard directories defined by the XDG Base Directory Specification; including respecting the definitions of the XDG_* environment variables when they are set by the user. These are the expected locations where applications ought to place files for a cohesive operating system, especially GNU+Linux-based operating systems.

https://specifications.freedesktop.org/basedir-spec/basedir-spec-latest.html