Add MIME apps spec utilities

Add MIME apps spec utilities

Facilitates finding associations between MIME types and desktop files
that report an association with that type.  Combined with mailcap.el's
MIME facilities, it should be easy to use desktop files.
* lisp/xdg.el (xdg-mime-table): New variable.
(xdg-mime-apps-files, xdg-mime-collect-associations, xdg-mime-apps):
New functions.
* test/data/xdg/mimeapps.list: New file.
* test/data/xdg/mimeinfo.cache: New file.
* test/lisp/xdg-tests.el (xdg-mime-associations): New test.