Add AppData metadata for quassel, quasselclient.

Add AppData metadata for quassel, quasselclient.

The freedesktop.org AppData standard defines metadata for GUI
applications that wish to show up in various graphical software
centers, such as GNOME Software. See the specification for
more details: https://people.freedesktop.org/~hughsient/appdata/

Applications without metadata will not show up in GNOME Software,
and possibly other graphical package managers. As GNOME Software
is the recommended way on Fedora Workstation to install software,
this means that without these files users won't be able to find
quassel or quasselclient.

Fixes #1400. Resolves GH-216.

(cherry picked from commit 91cdf2d76f6b57de2860f3d632cd2a40c17fb0b4)