Font-lock FDO desktop files correctly

Font-lock FDO desktop files correctly

Single and double quotes do not have a special meaning in
desktop files.
https://standards.freedesktop.org/desktop-entry-spec/latest/
* etc/NEWS: Mention new mode.
* lisp/files.el (auto-mode-alist): Split out an entry for handling
the .desktop extension with conf-desktop-mode.
* lisp/textmodes/conf-mode.el (conf-desktop-font-lock-keywords): New
variable with rules for booleans and format specifiers.
(conf-unix-mode): Remove desktop file entry example from docstring.
(conf-desktop-mode): New derived major mode.