Support XDG Base Directory specification on Linux

[witchymary]

I simply fixed merge conflicts and compilation issues from wangqr#132. Closes #122. Here's the original description by @WhiredPlanck:

This PR will fix Issue Aegisub#226. Inspired by or adapted from aria2/aria2@8bc1d37 and Aegisub#148.

My basic idea just is:

* If `$HOME/.aegisub` exists, then the program will still read/write config/cache/data from there;

* If `$HOME/.aegisub` doesn't exist, then the program will read/write config/cache/data from corresponding XDG Base Directory.

I think this idea may need more optimization, but I just pull a request first so we could discuss the detail.

[witchymary]

Note, this currently breaks DependencyControl due to https://github.com/TypesettingTools/DependencyControl/blob/master/modules/DependencyControl/Record.moon#L44. A fix would be to simply undo this small excerpt, but that kinda defies the point of the PR.

-	auto path_log = config::path->Decode("?user/log/");
+	auto path_log = config::path->Decode("?state/log/");

[heinrich5991]

Permalink: https://github.com/TypesettingTools/DependencyControl/blob/9b8d615f18e295f4777ea606a7173465bdaf140d/modules/DependencyControl/Record.moon#L44.