BUG: WIP: fix vtkPickingManager interaction with widgets

Adds options to explicitly enable and disable vtkAbstractPickers
so that the can be excluded from vtkPickingManager evaluations.

Updates vtkWidgetRepresentation so it can be told by when to
enable and disable any internally managed pickers.

Fixes the one class of pickers used in slicer so that version
4.4 can be released.

WIP: The changes in this commit should be evaluated in terms
of the vtk widget design and possibly picking can be automatically
handled as a byproduct of other calls such that EnablePicking and
DisablePicking are not needed in the public API.

See [1] for more discussion of the problem, along with a python
script that replicates the core issue that caused problem in slicer,
but using pure VTK.

http://na-mic.org/Bug/view.php?id=3808
Merge pull request #4 from pieper/3808-fiducial-picking

BUG: WIP: fix vtkPickingManager interaction with widgets