Currently the thumbnails are currently created in an internal function in QmitkNodeSelectionButton.cpp. That leads to unnecessary dependencies of QmitkNodeSelectionButton.cpp (e.g. to MultiLabel).

It would be cleaner to do the following:

1. add a virtual GenerateThumbnail method to QmitkNodeDescriptor
2. Base implementation should return for images the image thumbnails and for everything else the icon (as it is done already in the current code)
3. Add dedicated descriptor for MultiLabel that implements its own code (using the segmentation part of the current implementation)
4. Remove the multilabel include form QmitkNodeSelectionButton.cpp
5. Remove the MitkMultiLabel module dependency from MitkQwidgets