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/cmakeprojectmanager/cmakesettingspage.cpp

@@ -237,7 +237,7 @@ void CMakeToolItemModel::removeCMakeTool(const Core::Id &id)
     CMakeToolTreeItem *treeItem = cmakeToolItem(id);
     QTC_ASSERT(treeItem, return);
 
-    removeItem(treeItem);
+    takeItem(treeItem);
     m_removedItems.append(id);
 
     delete treeItem;
@@ -248,7 +248,7 @@ void CMakeToolItemModel::apply()
     foreach (const Core::Id &id, m_removedItems)
         CMakeToolManager::deregisterCMakeTool(id);
 
-    foreach (auto item, treeLevelItems<CMakeToolTreeItem *>(2)) {
+    foreach (auto item, itemsAtLevel<CMakeToolTreeItem *>(2)) {
         item->m_changed = false;
 
         bool isNew = false;
@@ -300,7 +300,7 @@ void CMakeToolItemModel::setDefaultItemId(const Core::Id &id)
 QString CMakeToolItemModel::uniqueDisplayName(const QString &base) const
 {
     QStringList names;
-    foreach (CMakeToolTreeItem *item, treeLevelItems<CMakeToolTreeItem *>(2))
+    foreach (CMakeToolTreeItem *item, itemsAtLevel<CMakeToolTreeItem *>(2))
         names << item->m_name;
 
     return ProjectExplorer::Project::makeUnique(base, names);