Dendrogram functionality has been moved to vtkDendrogramItem
and heatmap functionality has been moved to vtkHeatmapItem.
This makes it easier for us to arrange dendrograms & heatmaps
as we see fit within a context scene.

vtkTreeHeatmapItem now contains an object from both of these
new classes.  The vtkTreeHeatmapItem still handles some interaction
between these two objects.  For example, when a subtree is collapsed,
the vtkTreeHeatmapItem tells its heatmap to hide the rows that
correspond to the newly pruned leaves of the dendrogram.

This commit also adds the concept of orientation to all of these
classes.  For example, this allows us to draw heatmaps or dendrograms
"upside down" or "backwards" (relative to their default orientation).

Change-Id: Ibf3f6180c7f5a6806e4d8282e76e01e21258c8d8