<para>, <list> and <item> markup are not interpreted by Qt (at least 5.5)

Summary:
In the availability widget for instant preview, the tooltip doesn't actually show items as a list. when there's multiple items, the words aren't even separated by spaces.

this patch turns the limiters or blockers into a bulleted list.

Reviewers: Krita:_next, abrahams

Reviewed By: Krita:_next, abrahams

Subscribers: abrahams, rempt

Projects: Krita:_next

Differential Revision: https://phabricator.kde.org/D616