BUG: use the superclass slice node

Keeping a local slice node was a leftover from
the Annotations module where it kept a local
pointer to a slice node to differentiate between
being used as a 3d or 2d displayable manager.

Issue #1690
BUG: observe the slice composite node to get layout changes

Added a vtkInternal class to keep a weak pointer to the slice
composite node, add observers to it so that when the layout
changes the Markups widgets can be updated.
Following the Crosshair displayable manager template.
Issue #1690