Fix segmentation serialization by changing serializer name back

Summary: Rename MultiLabelSegmentationSerializer back to LabelSetImageSerializer so it is correctly found when serializing a segmentation. I really only changed the class name, not the file, since the file name still reflects the direction we want to go. If you think it should be completely reverted, please let me know.

Test Plan: Load any image, create a segmentation, save project. The saved project should be loadable and include the segmentation.

Reviewers: O1 MITK Reviewer Group I, floca

Reviewed By: O1 MITK Reviewer Group I, floca

Maniphest Tasks: T29547

Differential Revision: https://phabricator.mitk.org/D820