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.
We should change the name and comments about layout changes for this, since there's now no reason to refresh observers.  The observation could be made in the __init__ code directly.