Godot: [TRACKER] DirAccess bugs

This issue is meant to list all bugs to DirAccess/the Directory GDScript API, so that they can be investigated all at once:

• [x] #1680 Inconsistent support for user:// vs res:// directory operations
• [x] #1872 DirAccess fails on make_dir and dir_exists with user://
• [x] #1938 No ../ when entering the folder without permissions to read
• [x] #4617 Directory::get_space_left() does not return consistent "failure" values
• [x] #4705 Directory.dir_exists() always fails on res:// type paths even though Directory.open() works

See also discussion about paths in #4700.

_Existing PRs:_

• #584 Globals::localize_path was calling change_dir to check if path exists