When adding a vtkPicker to the picking manager, a vtkObject can optionally be
associated with the picker. This commit changes the behavior of
vtkPickingManager to not own (i.e. increment the reference count) of these
associated objects. Instead, the object references are held in a raw pointer.

The widget framework is the primary client of the picking manager. The typical
vtkWidgetRepresentation subclass adds its pickers to the picking manager and
includes an associated object reference to itself:

    pickingManager->AddPicker(picker, this);

When the widget/representation are destroyed, the vtkWidgetRepresentation
destructor unregisters its pickers:

    pickingManager->RemoveObject(this);

However, because the picking manager took ownership of the associated objects,
the vtkWidgetRepresentation instances and their pickers are not destroyed until
the vtkPickingManager is destroyed. In particular applications, this could lead
to a large number of objects referenced only by vtkPickingManager that would
exist until application exit.

The picking manager test is updated for the use case demonstrated by the widget
framework.