ENH: Improved segment editor widget 2D representation selection logic

Switch to use binary labelmap representation for 2D dispay only if this representation exists.
This is necessary to prevent switching representation when only ribbon and closed surface representation exists (for example, after DICOM-RT import).

git-svn-id: http://svn.slicer.org/Slicer4/trunk@25928 3bd1e089-480b-0410-8dfb-8563597acbee