make it easier to change dendrogram edge width

Extend the API of vtkDendrogram & related classes so that it is easier
to change how wide the edges of the trees are drawn.

Change-Id: I6a6689319547c07d496860c9bb78ce836f21b0b3
Update TestTreeHeatmapItem for line width changes

As tree edge width is no longer hardcoded, we need to update this
affected test so that its resulting image remains the same.

Change-Id: I28980f6348646d8c7ec6c986eab846b86ac21dcd
Merge topic 'set_tree_edge_width' into master

02f7a43f Update TestTreeHeatmapItem for line width changes
98a5b2a1 make it easier to change dendrogram edge width