ProjectExplorer: Nodes: Do not makeEmpty() as part of buildTree()

Do not call makeEmpty() as part of buildTree(). That makes it
impossible to combine buildTree with manual tree setup.

Change-Id: If0a0d9432fe39870917a6ba31594e8dcd6d31868
Reviewed-by: hjk <hjk@qt.io>

src/plugins/genericprojectmanager/genericproject.cpp

@@ -282,6 +282,7 @@ void GenericProject::refresh(RefreshOptions options)
                                               FileType::Project,
                                               /* generated = */ false);
         fileNodes << projectFilesNode << projectIncludesNode << projectConfigNode;
+        rootProjectNode()->makeEmpty();
         rootProjectNode()->buildTree(fileNodes);
     }