ENH results of merge with 6049:6469 from branches/tgl_loadable_modules.  Adds GenerateLM to create entry points for loadable module support via a text file and CMake macro. WFEngine, NeuroNav, Qdec and GAD are setup, other modules have text files, but the macro in the CMakeLists file is commented out.  Updated Slicer3_main,  the modules above will only be loaded if the DLL is found at run-time.  Lastly made a change to vtkSlicerViewerWidget, check for a NULL ModelHierarchyLogic pointer before continuing, which might not be the right thing to do.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@6570 3bd1e089-480b-0410-8dfb-8563597acbee