ENH: Interaction for transform node

The transform will automatically be centered around whatever objects
it transforms at the moment when the TransformUpdateEditorBoundsEvent is
invoked. This is more intuitive to the user as it looks as if the object under
the transform itself is moved/scaled/translated.
When no object is present, the transform is just scaled to facilitate
interaction with it.

Testing is added to ensure that:

• The widget holds correctly the objects it will transform. Furthermore this
  test is perfomed with models to extend the coverage as transform interaction
  can theoritically deal with any vtkMRMLDisplayableNode.
• Testing for interacting with transforms with parents
• Serialization and deserialization of the scene

The transform interaction is only valid linear transforms

From: Johan Andruejol <johan.andruejol@kitware.com>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@25698 3bd1e089-480b-0410-8dfb-8563597acbee