- introduce topLevelItemCount (similar to QTreeWidget)
- squash untypedTreeLevelItems()
- rename removeItems() to clear()
- rename removeItem() to takeItem()
- rename treeLevelItems<> to itemsAtLevel<>

Change-Id: I0f1bb4110f7687b20da3d92e3d943858645a9fa2
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>