Added Link button in 3DViewController; Added MRMLViewLogic: set the interactions; Added MRMLViewLinkLogic: propagate the events.
next step for Slicer 4.10 (no backward compatibility): merge vtkMRMLCameraNode into vtkMRMLViewNode
see also https://github.com/NA-MIC/ProjectWeek/blob/master/PW28_2018_GranCanaria/Projects/3DViewsLinking/README.md#progress-and-next-steps point (a)