emacs/desktop: update to use notmuch-emacs-mua and handle mailto

With the mailto: handling in notmuch-emacs-mua, we can update the
desktop file to advertize we can be set as the default application to
handle email. While at it, add GenericName and Comment to be more
informative.

With --hello, notmuch-emacs-mua will run (notmuch) if mailto: url is
not given.