AppStream - add launchable information

Quoted from https://appstream.debian.org/sid/main/issues/plasma-discover.html :

asv-desktop-app-launchable-omitted

This desktop-application component has no desktop-id launchable tag, however it contains all the necessary information to display the application. The omission of the launchable entry means that this application can not be launched directly from installers or software centers. > If this is intended, this information can be ignored, otherwise it is strongly recommended to add a launchable tag as well.

This patch tries to address this issue by adding such information.