Improve handling of <em> and <code> tags

[heftig]

Three commits:

• Properly initialize is_{em,code}_text fields Fixes: https://github.com/hughsie/appstream-glib/issues/445
• trivial: Turn is_{em,code}_text fields into bitfields

• Improve handling of \<em> and \<code> tags

  This is still not great code but at least somewhat an improvement. Tests were expanded to showcase the new behavior.

  I think, ideally, we would append opening/closing tags to the ancestor p or li node's cdata as soon as we encounter the start/end of an em or code element. This would then also handle empty elements correctly.

[hughsie]

Thanks!