Use specific autosave dir & dont try to open directories

[wpkelso]

Fixes #20 , #21

[ryonakano]

There are some leftovers in Window.vala, don't we need to replace them to Application.data_dir_path? 🤔

ryo@b760m:~/work/tmp/slate (autosave-dir =)$ git grep Environment
src/Application.vala:    public static string data_dir_path = Environment.get_user_data_dir () + "/slate";
src/Window.vala:        bool delete_after = (Environment.get_user_data_dir () in this.file.get_path ());
src/Window.vala:        bool is_unsaved_doc = (Environment.get_user_data_dir () in this.file.get_path ());
ryo@b760m:~/work/tmp/slate (autosave-dir =)$

[wpkelso]

There are some leftovers in Window.vala, don't we need to replace them to Application.data_dir_path? 🤔

Ah, yeah, looks like I missed those two 😅

[ryonakano]

Thanks for the fix, almost LGTM but I proposed #23 for an edge case.