QmlDesigner.propertyEditor: style fix

I moved the tooltip into the parent widget to avoid coloring issues.

Task-number: QTCREATORBUG-5480
Change-Id: Ic7f70d83e22847ca8f257be3728139f68a0cb77c
Reviewed-on: http://codereview.qt.nokia.com/1776


Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Thomas Hartmann <Thomas.Hartmann@nokia.com>