TaskTree: Get rid of Storage subclass of GroupItem

Make the c'tor of the GroupItem taking the TreeStorageBase
public instead.

This addresses the 20th point in the master task below.

Task-number: QTCREATORBUG-28741
Change-Id: I78320ec24b4e4b915ab549135fb93b45c658b9f2
Reviewed-by: hjk <hjk@qt.io>

src/plugins/autotest/testrunner.cpp

@@ -446,7 +446,7 @@ void TestRunner::runTestsHelper()
         };
         const Group group {
             finishAllAndSuccess,
-            Tasking::Storage(storage),
+            storage,
             onGroupSetup(onSetup),
             ProcessTask(onProcessSetup, onProcessDone)
         };