Add .desktop and .appdata.xml descriptions

Hi,
this is more of an RFC than a real pull request. I have been working on a Fedora package of Luppp (https://bugzilla.redhat.com/show_bug.cgi?id=1043573), and this adds the integration in the application menu and gnome-software (a graphical tool to install applications).

I know the icon is not particularly pretty... so it would be great if you replaced it with something better. But the .desktop and .appdata.xml files should be OK.