ENH: Added mouse cursor position to vtkMRMLCrosshairNode.

Now cursor (now only mouse position in 2D Slice view, but later also other pointing devices can be supported) position can be obtained by observing the vtkMRMLCrosshairNode singleton node.

The feature is used in:
* DataProbe window
* Transform info window (showing the transform displacement vector value at the current mouse position in real-time)

After the integration into the Slicer core is done, it will be also used in MultiVolume, Multidimensional data for live 3D graphing.
I am assuming you mean  `we need to handle EnterEvent here`, the code is not really adding an observer.