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/debuggertooltipmanager.cpp

@@ -199,7 +199,7 @@ public:
     ToolTipWatchItem(WatchItem *item);
 
     bool hasChildren() const { return expandable; }
-    bool canFetchMore() const { return children().isEmpty() && expandable && model(); }
+    bool canFetchMore() const { return childCount() == 0 && expandable && model(); }
     void fetchMore() {}
     QVariant data(int column, int role) const;