TaskTree: Introduce onGroup...() functions

The OnGroup... elements are going to be replaced with
these functions.

Change-Id: Ia271a89062cc9c6b18384607b20b1f68d273bcde
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: hjk <hjk@qt.io>

src/libs/solutions/tasking/tasktree.cpp

@@ -40,6 +40,16 @@ private:
     Guard &m_guard;
 };
 
+TaskItem onGroupDone(const TaskItem::GroupEndHandler &handler)
+{
+    return Group::onGroupDone(handler);
+}
+
+TaskItem onGroupError(const TaskItem::GroupEndHandler &handler)
+{
+    return Group::onGroupError(handler);
+}
+
 static TaskAction toTaskAction(bool success)
 {
     return success ? TaskAction::StopWithDone : TaskAction::StopWithError;