* lisp/xdg.el: Add support for Desktop Entry Specification.
(xdg--user-dirs-parse-line): Check if file is readable.
(xdg-desktop-group-regexp, xdg-desktop-entry-regexp): New variables.
(xdg--desktop-parse-line, xdg-desktop-read-file, xdg-desktop-strings):
New functions.
* test/lisp/xdg-tests.el:
* test/data/xdg/test.desktop:
* test/data/xdg/wrong.desktop: New files.
