[AppStream] Additional markup support in descriptions

[Matthias Klumpp]

Hey :)

This feature is now implemented[1] and documented in the spec[2]. You
can use <em/> and <code/> in paragraphs and list items in AppStream
descriptions.
This change turned out to actually be a bit more invasive than I
thought, it not only required changes in the parser code and
validator, but also needed changes to the ITS set.
If you are using libappstream, there is actually a bit of forward
compatibility already built-in when reading files that contain the new
tags with older AppStream versions (they'd get dropped, but the text
itself is preserved, just without the extra markup). However, writing
them will not work, and since libas rewrites the XML for caching, text
might have the marked-up text pieces missing when loaded with older
AppStream versions.

So, please don't use the feature everywhere just yet until AppStream
0.12.8 had at least a little bit of time to get widely used by
distributions, or you can guarantee that the AppStream version on the
receiving end is recent enough.

Cheers,
    Matthias

[1]: https://github.com/ximion/appstream/commit/89d92c914e1cc328c8bfef51edfd07a809d5aff8
[2]: https://github.com/ximion/appstream/commit/e774931febe4ea8a85504e4f825626907f2020ae

-- 
I welcome VSRE emails. See http://vsre.info/