ENH: Add members to enable switching between fractional labelmap display types

When needed later, accessors can be added to change these members to switch from the default thresholded contour visualization to the continuous opacity one for 2D segmentation fractional labelmap display

git-svn-id: http://svn.slicer.org/Slicer4/trunk@25319 3bd1e089-480b-0410-8dfb-8563597acbee