Add a dendrogram for the columns of the heatmap

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
add test for column trees

Add a test for the new column tree functionality of
vtkTreeHeatmapItem.

This commit also updates the test for vtkTanglegramItem.  The
appearance of this item has changed now that column labels are
included in the values returned by GetBounds().

Change-Id: I36b8383533d9c61db81e47df278d34577d6ddbeb
Merge topic 'collapse_columns' into master

8c716e6d add test for column trees
728192ea Add a dendrogram for the columns of the heatmap