Run strict validation on AppData file

[Stephan Bergmann]

An issue started to appear when I tried building a new version of 
LibreOffice for Flathub.  While recent test build 
<https://flathub.org/builds/#/builders/32/builds/3710> was successful 
(albeit with warnings), the corresponding official build 
<https://flathub.org/builds/#/builders/32/builds/3811> fails now due to 
validation errors at <https://flathub.org/builds/#/builders/24/builds/370>:

> flatpak run org.freedesktop.appstream-glib validate-strict builddir/*/share/appdata/org.libreoffice.LibreOffice.appdata.xml
>  in dir /srv/buildbot/worker/build-i386-3/build (timeout 1200 secs)
>  watching logfiles {}
>  argv: b'flatpak run org.freedesktop.appstream-glib validate-strict builddir/*/share/appdata/org.libreoffice.LibreOffice.appdata.xml'
>  using PTY: False
> builddir/files/share/appdata/org.libreoffice.LibreOffice.appdata.xml: FAILED:
> • tag-missing           : <translation> not specified
> • value-missing         : <!-- Copyright [year] [name] --> is not present
> • aspect-ratio-invalid  : <screenshot> aspect ratio not 16:9 [https://hub.libreoffice.org/screenshots/writer-01.png]
> • aspect-ratio-invalid  : <screenshot> aspect ratio not 16:9 [https://hub.libreoffice.org/screenshots/calc-02.png]
> • aspect-ratio-invalid  : <screenshot> aspect ratio not 16:9 [https://hub.libreoffice.org/screenshots/impress-01.png]
> • aspect-ratio-invalid  : <screenshot> aspect ratio not 16:9 [https://hub.libreoffice.org/screenshots/draw-02.png]
> • aspect-ratio-invalid  : <screenshot> aspect ratio not 16:9 [https://hub.libreoffice.org/screenshots/base-02.png]
> • translations-required  : <name> has no translations
> • translations-required  : <summary> has no translations
> • translations-required  : <description> has no translations
> Validation of files failed
> program finished with exit code 1
> elapsedTime=3.802360

The previous test build had failed hard due to a missing content_rating 
element in the AppData file.  I complained about that rather surprising 
change of things and thought that I was done for now with massaging 
LibreOffice's AppData after I had added that.  But apparently I was wrong.

Now, I can probably add a copyright comment and do something about the 
screenshot aspect ratios, but the rest of the complaints above leave me 
clueless:

LibreOffice does internally use gettext by now, and it contains various 
/app/share/runtime/locale/*/resource/*/LC_MESSAGES/*.mo files, so I 
should probably add a translation element with attribute type="gettext". 
  But what should the content of that element be? 
<https://www.freedesktop.org/software/appstream/docs/chap-Metadata.html#sect-Metadata-GenericComponent> 
is completely silent on that topic and just gives an unhelpful 
"<translation type="gettext">foobar</translation>" example, while 
<https://github.com/flathub/flathub/wiki/AppData-Guidelines#translations> 
states that "you’ll need to provide the prefix of these files with a 
<translation/> tag"---whatever "the prefix of these files" is meant to be.

For the "translation-required" of the name, summary, and description 
elements, how would I even specify such translations?  An additional 
name element with an xml:lang="..." attribute? 
<https://www.freedesktop.org/software/appstream/docs/chap-Metadata.html#sect-Metadata-GenericComponent> 
again appears to be completely silent on that, as does 
<https://github.com/flathub/flathub/wiki/AppData-Guidelines>.