Revert "Speed up QmlGraphicsItem::setParentItem."
Revert "Speed up QmlGraphicsItem::setParentItem."

This reverts commit 282a46ab5edafa7b82e9c58658143cb979db2d85.

This change introduced some regression due to itemChange notifications
not being delivered as expected (reported by Warwick). We have to find
another solution, unfortunately. However, the performance penalty isn't
that big so we can live with it for now.

pruiz/wkhtmltopdf-qt,pruiz/wkhtmltopdf-qt,pruiz/wkhtmltopdf-qt,igor-sfdc/qt-wk,igor-sfdc/qt-wk,pruiz/wkhtmltopdf-qt,igor-sfdc/qt-wk,igor-sfdc/qt-wk,igor-sfdc/qt-wk,pruiz/wkhtmltopdf-qt,pruiz/wkhtmltopdf-qt,pruiz/wkhtmltopdf-qt,pruiz/wkhtmltopdf-qt,igor-sfdc/qt-wk,pruiz/wkhtmltopdf-qt,igor-sfdc/qt-wk,igor-sfdc/qt-wk,pruiz/wkhtmltopdf-qt,pruiz/wkhtmltopdf-qt,igor-sfdc/qt-wk,igor-sfdc/qt-wk,igor-sfdc/qt-wk