kdoctools: style images more elegantly

The ugly <hr> elements that used to surround DocBook <mediaobject>
elements are removed, and replaced with CSS styling around
<screenshot> and <informalfigure> elements only.  Plain
<mediaobject> elements are now unstyled.

As discussed on the documentation team mailing list:
http://lists.kde.org/?l=kde-doc-english&m=141019053410846&w=2

The change will be forward-ported to kdoctools (KF5).

REVIEW: 120210