Test interactively the functionalities with the example:
- TestPickingManagerWidgets.cxx
- TestPickingManagerSeedWidget.cxx

Unit test added for the internal data structure && performances:
- TestPickingManager.cxx
- TestPickingManagerSeedWidget.cxx (Perfs)

The management is currently implemented and enabled by default
with the following widgets:
- vtkBoxWidget
- vtkBoxWidget2
- vtkBrokenLineWidget
- vtkBalloonWidget
- vtkEllipsoidTensorProbeRepresentation
- vtkImagePlaneWidget
- vtkImageTracerWidget
- vtkImplicitPlaneWidget
- vtkLineWidget
- vtkLineWidget2
- vtkPlaneWidget
- vtkProp3DButtonRepresentation
- vtkSphereHandleRepresentation
- vtkSphereWidget
- vtkSphereWidget2
- vtkSplineWidget
- ...

This effort is part of the MSV project.

 Change-Id: I9a92bed7c36903f56835b7e737f2aea6b7c1fc44

Minor comment fixes to be merged within existing commit

Change-Id: I5b25d9d9d960e5a5dea332c7af2a20a4e9756c28