Support XDG Base directory specification for Linux

[WhiredPlanck]

This PR will fix Issue Aegisub#226. Adapted from aria2/aria2@8bc1d37.

The basic idea just is:

• If $HOME/.aegisub exists, then the program will still read/write config/cache/data/state from there;
• If $HOME/.aegisub doesn't exist, then the program will read/write config/cache/data/state from corresponding XDG base Directory.

The original PR is here: wangqr#132

[witchymary]

I'm pretty sure this will still run into the issues I fixed here: arch1t3cht@b5a071b

Also worth noting that this will break regardless due to this line in Dependency Control (https://github.com/TypesettingTools/DependencyControl/blob/9b8d615f18e295f4777ea606a7173465bdaf140d/modules/DependencyControl/Record.moon#L44). It needs to be changed in there as well

[WhiredPlanck]

I'm pretty sure this will still run into the issues I fixed here: arch1t3cht@b5a071b

Also worth noting that this will break regardless due to this line in Dependency Control (https://github.com/TypesettingTools/DependencyControl/blob/9b8d615f18e295f4777ea606a7173465bdaf140d/modules/DependencyControl/Record.moon#L44). It needs to be changed in there as well

You're right, I will fix it.