TreeView: allow rows to expand even without children

Current behavior:

In it's current state the TreeView doesn't seem to allow a row to change it's expansion state unless it has children.

=> This forces the user to wait for the first child to be added to the row before being able to mark the row as expanded.

Desired behavior:

A row can be expanded regardless of whether it has children or not.

=> User can mark a row as expanded without having to wait and listen for the first child to be added.