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/qmlprojectmanager/qmlproject.cpp

@@ -396,8 +396,9 @@ void QmlProject::generateProjectTree()
         return new FileNode(Utils::FileName::fromString(f), fileType, false);
     });
 
-    rootProjectNode()->makeEmpty();
-    rootProjectNode()->buildTree(fileNodes);
+    auto newRoot = new Internal::QmlProjectNode(this);
+    newRoot->buildTree(fileNodes);
+    setRootProjectNode(newRoot);
 }
 
 } // namespace QmlProjectManager