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/nim/project/nimproject.cpp

@@ -156,7 +156,7 @@ void NimProject::updateProject()
 
     auto newRoot = new NimProjectNode(*this, projectDirectory());
     newRoot->setDisplayName(displayName());
-    newRoot->buildTree(fileNodes);
+    newRoot->addNestedNodes(fileNodes);
     setRootProjectNode(newRoot);
 
     emit fileListChanged();