vtkRenderedTreeAreaRepresentation (and hence vtkTreeRingView)
no longer use vtkQtTreeRingLabelMapper to render labels in a
tree ring bounded by the tree regions and with ellipsis.
This functionality is rolled into vtkLabelPlacementMapper.
This required several changes:

 - vtkStackedTreeLayout produces a "BoundedSize" array that
   is the maximum length and width of a centered label that will
   fit in the region.

 - vtkPointSetToLabelHierarchy and vtkLabelHierarchy now can store
   the "BoundedSize" array.

 - vtkLabelPlacementMapper checks whether this array exists, and
   if the label rendering strategy supports it, and if so does
   attempts to place each label within its region.

 - vtkLabelRenderStrategy has a new method SupportsBoundedSize()
   that returns true for Qt rendering and false for FreeType rendering.

 - vtkQtLabelRenderStrategy supports rendering bounded size labels
   with ellipsis (...).