TaskTree: Rename TaskItem into GroupItem

Not all classes derived from TaskItem are tasks,
but the common denominator is that all may be placed
inside a group: thus GroupItem sounds more appropriate.

Addresses the 10th point in the bugreport below.

Task-number: QTCREATORBUG-28741
Change-Id: I94d728a8e39ec732810f2e5bbe6b9a76f3bc387c
Reviewed-by: Qt CI Patch Build Bot <ci_patchbuild_bot@qt.io>
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: hjk <hjk@qt.io>

src/plugins/autotest/testrunner.cpp

@@ -347,7 +347,7 @@ void TestRunner::runTestsHelper()
         std::unique_ptr<TestOutputReader> m_outputReader;
     };
 
-    QList<TaskItem> tasks{finishAllAndDone};
+    QList<GroupItem> tasks{finishAllAndDone};
 
     for (ITestConfiguration *config : m_selectedTests) {
         QTC_ASSERT(config, continue);