ProjectExplorer: Add a FolderNode::addNestedNode() function

Also, rename buildTree() to addNestedNodes(), it's really
adding things, not recompletely (re-)building the subtree.

Use it whenever possible to avoid intermediate lists of items
to insert.

Change-Id: I5fde41e5b164a8a292410bd381a85f5efadf3471
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>

src/plugins/cmakeprojectmanager/tealeafreader.cpp

@@ -311,7 +311,7 @@ void TeaLeafReader::generateProjectTree(CMakeListsNode *root, const QList<const
 
     QList<FileNode *> fileNodes = m_files + Utils::transform(missingHeaders, [](const FileNode *fn) { return new FileNode(*fn); });
 
-    root->buildTree(fileNodes, m_parameters.sourceDirectory);
+    root->addNestedNodes(fileNodes, m_parameters.sourceDirectory);
     m_files.clear(); // Some of the FileNodes in files() were deleted!
 }