Automatically expand single-element in calltree view of tree explorer

The calltree view is cool *but* it doesn't do a good job of expanding eagerly in the case where there's only a single possible child. We should eagerly expand on every entry as long as there's only one child.

Start by locating the calltree view in the tree explorer and tracing how entries with children are expanded. Verify the behavior with a call tree where entries have a single child, ensuring expansion continues through single-child entries and stops when an entry has multiple children.

Written by the indexing model from the issue text.