TreeModel: More API cleanup

- 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>

src/plugins/debugger/debuggerengine.cpp

@@ -1944,7 +1944,7 @@ void DebuggerEngine::updateLocalsView(const GdbMi &all)
 
     QSet<QByteArray> toDelete;
     if (!partial) {
-        foreach (WatchItem *item, handler->model()->treeLevelItems<WatchItem *>(2))
+        foreach (WatchItem *item, handler->model()->itemsAtLevel<WatchItem *>(2))
             toDelete.insert(item->iname);
     }