Add an AppData file

See http://people.freedesktop.org/~hughsient/appdata/ for details.

https://bugzilla.gnome.org/show_bug.cgi?id=708757
Use intltool's GSettings support

Necessary to be able to build the AppData XML in the same folder.

https://bugzilla.gnome.org/show_bug.cgi?id=708757