Replace fugly workaround for QTBUG-6322 by one that's much simpler

Replace fugly workaround for QTBUG-6322 by one that's much simpler

Since it doesn't seem to matter at all what ChatView thinks about the scene's width,
we just adjust the sceneRect to be 25ps smaller than it really is. That way, we avoid
the whole QTBUG-6322 mess and most probably don't break anything.

Yes, zooming works again as well/