vtkTreeHeatmapItem now supports a dendrogram for the columns of the
heatmap.  This provides users with a way to interactively collapse
and expand columns.

The GetBounds() methods of vtkDendrogram, vtkHeatmapItem, and
vtkTreeHeatmapItem now include the size of any displayed text labels.
This required some changes to the way that the components of
vtkTanglegramItem are laid out.

Change-Id: Icdc9342d360b772819ff7863ed2a971e8865003e