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.

Also separated SetCursorPositionRAS and SetCursorPositionXYZ methods to
allow 3D-only positioning devices to specify the cursor position
(when no 2D slice position is available)

From: Andras Lasso <lasso@queensu.ca>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23387 3bd1e089-480b-0410-8dfb-8563597acbee