Make all projects generate a fresh project tree

Do not update the existing project tree anymore: Start a fresh one
and throw the old one away.

Change-Id: Ifabe293b6ca668b0672516a6d81acd5346d98fe5
Reviewed-by: hjk <hjk@qt.io>

src/plugins/genericprojectmanager/genericproject.cpp

@@ -278,8 +278,10 @@ void GenericProject::refresh(RefreshOptions options)
                                               FileType::Project,
                                               /* generated = */ false);
         fileNodes << projectFilesNode << projectIncludesNode << projectConfigNode;
-        rootProjectNode()->makeEmpty();
-        rootProjectNode()->buildTree(fileNodes);
+
+        auto newRoot = new GenericProjectNode(this);
+        newRoot->buildTree(fileNodes);
+        setRootProjectNode(newRoot);
     }
 
     refreshCppCodeModel();