ENH: Add option for exporting DICOM segmentation objects from editor

This commit transitions some functionality that was buried in
the LabelStructureListWidget into the EditUtil class so that it
can be used in other parts of the code (in particular in the
DICOMSegmentationPlugin provided by the Reporting extension).

This code is in a transitional state where the core depends
on code from the extension (the EncodeSEG CLI module). Once
the SEGSupport converters are moved to the core that dependency
will go away.

From: Steve Pieper <pieper@isomics.com>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@24769 3bd1e089-480b-0410-8dfb-8563597acbee