Update and fix our .desktop file.

Update and fix our .desktop file.

	* etc/sxemacs.desktop.in (StartupNotify): Set to false.  I'm
	almost certain we don't do this.

	* etc/sxemacs.desktop.in (MimeType): Update to include all the
	MIME types I can think of that we support.

	* etc/sxemacs.desktop.in (Categories): The spec says that if we're
	a "TextEditor", we should also be a "Utility".  So add that.

Signed-off-by: Steve Youngs <steve@sxemacs.org>