Generalize the HTML warning box for inline messages
Summary:
This follows what KMessageWidget or Kirigami.InlineMessage do, making this
visually and conceptually more consistent with everything else.
In here we only need the error message box (for the HTML warning), the
other ones are going to be used by plugins.
While at it, align the visual style for <hr> with Kirigami.Separator.
Reviewers: knauss, mlaurent
Reviewed By: knauss, mlaurent
Subscribers: kde-pim
Tags: KDE PIM
Differential Revision: https://phabricator.kde.org/D23299