Add support for XDG base directory.

[c4llv07e]

I have a non-standard config and cache location, and I would like Abaddon to follow the XDG standards and try to use these variables for default directories.

This does not affect normal users, because they either have these variables pointing to .config, .cache and .local/share or they simply don't have it.

Maybe I should refactor this copy-pasted code, but I am not sure if this will make it better.

[ouwou]

the copy pasting definitely isnt the right move but i can go in and clean this up probably

[c4llv07e]

the copy pasting definitely isnt the right move but i can go in and clean this up probably

Something like this?

[c4llv07e]

I don't like the "" solution, but returning C strings is even worse. I can't find any std::string* in the project, so I assumed that there shouldn't be any allocations in the functions, and because of that I can't use nullptr either.

[c4llv07e]

Force pushed because of the rebase.