[AppStream] Adding URL kind "translation"

[Richard Hughes]

Hey,

I've been talking to a lot of translators at Flock this week. Without
many exceptions they've told me it's hard to translate AppData files
as they don't know where to submit translations just from looking at
the software center. I've been mostly successful saying "do it
upstream" again and again but we're not making it easy to discover
where "upstream" is for translations. I was thinking about proposing
an optional:

<url kind="translation">https://fedora.zanata.org/project/view/appstream-glib</url>

where the actual value in practice would typically be a Zanata or
Transifex instance. This can either be discovered using a widget in
the software center or just from a webapp that parses the AppStream
metadata (much like Jean-Baptiste has produced).

I guess this should be pretty uncontroversial -- but feedback very
welcome. If this gets blessed then I'll add support to appstream-glib
and then blog about it once the new tarball has hit the major distros
(to avoid validation failures).

Thanks,

Richard