Add appdata.xml file

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

Change-Id: Idb30ddd17b80a0f606aa42f062b76905f11446b5
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>