Bug#907756: freecol: multiple warnings from AppStream metadata

[Jeremy Bicha]

Source: freecol
Version: 0.11.6+dfsg2-2

There are still multiple issues with the Appstream metadata for
freecol. See the reference below.

In my testing, these issues seem to prevent FreeCol from showing up in
the GNOME Software app.

Honestly, I'm not too concerned about the reverse domain name style
for the id, but the other issues shouldn't be too difficult to solve.

update_contact
The screenshot should be in an image tag.

References
----------------
https://appstream.debian.org/sid/main/issues/freecol.html

https://www.freedesktop.org/software/appstream/docs/chap-Metadata.html#tag-screenshots

Thanks,
Jeremy Bicha