Fix 'On sequence diagrams no display of a selected object widget on the corresponding line'

If an object widget is selected, the corresponding sequence line is
displayed with a larger line width.

It was necessary to fix a problem with QGraphicsItem::setSelected() to
implement the requested behavior. Details can be found in the
documentation of the class QGraphicsObjectWrapper.

BUG:418249
FIXED-IN:2.30.80 (KDE releases 20.03.80)