ENH: Improve CLI image widget type handling

• show both LabelMap and scalar volume nodes if "scalar" type
  is specified (to restore behavior before vtkMRMLLabelMapVolumeNode
  was added)

• allow adding either vtkMRMLScalarVolumeNode or vtkMRMLLabelMapVolumeNode
  if "scalar" type is specified

• allow adding of multiple output node types if "any" type is specified
  as output

Reviewed-by: Jean-Christophe Fillion-Robin <jchris.fillionr@kitware.com>

From: Andras Lasso <lasso@queensu.ca>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@24351 3bd1e089-480b-0410-8dfb-8563597acbee