Linux filepaths do not comply with the XDG Base Directory Specification

[jamescrake-merani]

Describe the bug
This was raised during the review of the Flatpak. Right now, we are storing all of the configuration files, and compiled models that SasView generates in a directory that lives directly on the home directory. This does not comply with the XDG Base Directory Specification (a kind of standard for Linux desktop applications).

Configuration files should be placed under ~/.config/sasview, and user specific data should be stored in ~/.local/share/sasview. The latter should also included the sasview.log file which should not be on the home directory. The idea of this is to not pollute the user's home directory.

This is a low priority issue as it should hopefully not block the Flatpak from being published. In fact, many popular applications break this rule as well. But it is probably a change we'll want to make in the future to promote better practices.

If a fix were to be implemented, there is an issue of older installations using the old directories, and then these being missed by newer installations. To get around this, I would suggest having a precedence system where we check one location first before checking another directory. This is a common approach taken by many other Linux applications.

SasView version:

• Version: 6.0.0

Operating system:

• OS: Any Linux distribution