Root-level expandable item.
This item contains the first-level source list items. It only serves as an item container. It is used to add and remove items to/from the widget.
Internally, it allows the source list to connect to its Granite.Widgets.SourceList.ExpandableItem.child_added and Granite.Widgets.SourceList.ExpandableItem.child_removed signals in order to monitor new children additions/removals.