Added Appstream XML data for better Gnome and KDE integration.

Gnome is trying to get AppData XML file for applications so AppStream Gnome system would work. Fedora 22 plans to make applications that ship Appstream appdata first class apps. So here is XML file for Mixxx. Wording and stuff should be checked also is screenshot good enough.

Also should desktop and appdata only install when you on Linux or *BSD?
Some small nit pick.  Could you replace `Copyright` with `Author`? Would that be ok for you?