Blender Git Loki

Fix T48741: File browser back button doesn't work from inside Blend (library) file.

Problem was in fact slightly wider, File space was nearly not taking into account
library navigation case and its 'virtual' directoris, except in a few places.

Add a wrapper around BLI_is_dir that also check for lib paths, and used it in
ED_file_change_dir(), such that we now always check path is a
valid directory - in the filebrowser context, not filesytem context. ;)