Discover depending on unreleased Appstream

[Jonathan Riddell]

Plasma Discover recently got an update to use Appstream Qt API which
was itself only committed a few days ago

https://phabricator.kde.org/D6267 Use appstream-qt insted of glib based one

https://github.com/ximion/appstream/commit/548a5ae6617ef1c2523981cadb73fd2d41e0cf43#diff-e85823b4e7db1064f4301e1c74978199


Do we have any reassurances that appstream will be released and
realistic for distros to use before the Plasma 5.11 release?

Jonathan