An item that can contain more items.
It supports all the properties inherited from Granite.Widgets.SourceList.Item, and behaves like a normal item, except when it is located at the root level; in that case, the following properties are ignored by the widget:
* Granite.Widgets.SourceList.Item.selectable * Granite.Widgets.SourceList.Item.editable * Granite.Widgets.SourceList.Item.icon * Granite.Widgets.SourceList.Item.activatable * Granite.Widgets.SourceList.Item.badge
Root-level expandable items (i.e. Main Categories) are not displayed when they contain zero visible children.