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/qbsprojectmanager/qbsnodetreebuilder.h

@@ -39,7 +39,7 @@ namespace Internal {
 class QbsNodeTreeBuilder
 {
 public:
-    static void buildTree(QbsProject *project);
+    static QbsRootProjectNode *buildTree(QbsProject *project);
 };
 
 } // namespace Internal