ENH: Make layoutManager() method in qMRMLLayoutWidget Q_INVOKABLE

To make it accessible from python

From: Csaba Pinter <csaba.pinter@queensu.ca>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22856 3bd1e089-480b-0410-8dfb-8563597acbee