BUG: Ensure qMRMLVolumeInfoWidget state is cleared when widget is disabled.

See http://na-mic.org/Mantis/view.php?id=2881
Fixes #2881

git-svn-id: http://svn.slicer.org/Slicer4/trunk@21709 3bd1e089-480b-0410-8dfb-8563597acbee