The AppData files provide descriptive data about an application, and is
typically used in application managers to display additional information to
the user. Having an AppData file is especially important for some graphical
application installers: without one, an application manager may not event
show an entry for the application. This happens for example in GNOME Software.
For the sake of completeness, this commit not only adds the AppData file to
the Flatpak build, but also to the Linux .deb and .rpm packages.
