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/libs/extensionsystem/pluginview.cpp

@@ -392,7 +392,7 @@ PluginSpec *PluginView::pluginForIndex(const QModelIndex &index) const
 void PluginView::updatePlugins()
 {
     // Model.
-    m_model->removeItems();
+    m_model->clear();
 
     PluginCollection *defaultCollection = 0;
     QList<CollectionItem *> collections;