display number of leaf nodes in a collapsed tree

vtkDendrogramItem has a new option to draw the number of leaf nodes
hidden by a collapsed subtree's triangle.  This behavior is enabled
by default.

Change-Id: I929ea76a7d2353516f72903266d75ce1acfd0388
Merge topic 'display_number_of_leaf_nodes' into master

dcaed73d display number of leaf nodes in a collapsed tree