TaskTree: Prepare for de-utils-ization - part 2

Move TaskTree into Tasking namespace.
Move Tasking namespace out of Utils namespace.

Change-Id: Ib4c1d7f54f1808517e54768dfa27209c33517b61
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: hjk <hjk@qt.io>

src/plugins/coreplugin/progressmanager/taskprogress.cpp

@@ -14,6 +14,7 @@
 #include <QTimer>
 
 using namespace Utils;
+using namespace Tasking;
 
 namespace Core {