Add function to read all entries in a group
Use that to extend xdg-desktop-read-file. Also fix a bug where all
entries in all groups were read and returned by xdg-desktop-read-file.
* lisp/xdg.el (xdg-desktop-read-group): New function.
(xdg-desktop-read-file): Use it.
* test/data/xdg/malformed.desktop: New file.
* test/data/xdg/test.desktop: Add another section.
* test/lisp/xdg-tests.el (xdg-desktop-parsing): Test presence of a key
in another group. Test reading a prescribed group. Test detecting a
malformed key=value.