Add AppStream metadata

Please adjust the project's descriptions to your liking.
The project metadata was taken mainly from kde.org (and therefore might
need to be corrected there as well, in case it has flaws).
For more information on the metadata, see: http://lists.kde.org/?l=kde-
core-devel&m=140702291220410
If you want to extend the metadata, helpful information can be found at
http://www.freedesktop.org/software/appstream/docs/chap-Quickstart.html