Fix painting of search results

Painting the highlighting was wrong, and it was worst for selected rows.
The QItemDelegate::drawDisplay implementation also paints the
highlighted background at the wrong position wrt to the text
(PM_FocusFrameHMargin), so we now do all the highlights ourselves.

Task-number: QTCREATORBUG-9838
Change-Id: Iad68f114a984680e41aeb78d7c61a16651653213
Reviewed-by: David Schulz <david.schulz@digia.com>
Reviewed-by: Orgad Shaneh <orgads@gmail.com>