Support non-Steam installations of the game

[alterNERDtive]

Currently the scripts auto-detect the installation and appdata folders from your Steam library. Which is quite cool, if I do say so myself. But … it means they currently do not support the GoG version (and any potential other versions out there).

For now I will probably just add some environment variables or something that let you set the paths manually. Which can also be marginally useful even if you do play the game on Steam.

Since there are various ways to even install GoG games on Linux, I’ll probably not work on auto detection. If anyone wants to supply a pull request, I’m here for it.