Basically support XDG Base Directory specification on Linux

[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.

[WhiredPlanck]

I've tested this on my system, work as expect.

[polirritmico]

Thanks! work without problems.

[WhiredPlanck]

Thanks! work without problems.

You're welcome, and thanks for your test result too ~

[louisroyer]

Hi, thanks for this work.

Could you update your code to use $XDG_STATE_HOME (according to version 0.8 of basedir-spec) to store history/log/autoback/autosave instead of $XDG_CONFIG_HOME ?

Also for code clarity, I suggest you to rename ?localto ?cache and ?user to ?config (and to create ?state).

[WhiredPlanck]

Hi, thanks for this work.

Could you update your code to use $XDG_STATE_HOME (according to version 0.8 of basedir-spec) to store history/log/autoback/autosave instead of $XDG_CONFIG_HOME ?

Also for code clarity, I suggest you to rename ?localto ?cache and ?user to ?config (and to create ?state).

Sure. But now I am too busy to work on this. I will make my way to update the code. Anyway, thanks for your review and suggestions ~

[WhiredPlanck]

Could you update your code to use $XDG_STATE_HOME (according to version 0.8 of basedir-spec) to store history/log/autoback/autosave instead of $XDG_CONFIG_HOME ?

@louisroyer I finish this in commit 6fba9a6.