Include timestamp in QSinglePointEvent debug streaming operator

It's sometimes useful. QEventPoint debug output includes the timestamp;
but for ordinary mouse events, we're often focused on the event itself
rather than its eventpoint.

Change-Id: Ib65922331c8601bbed5aea3ea96c820906c89ef2
Reviewed-by: Liang Qi <liang.qi@qt.io>
(cherry picked from commit ac1ee25bcfe812adb9d57ee0b78214d91223fde3)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/gui/kernel/qevent.cpp

@@ -4077,6 +4077,7 @@ QDebug operator<<(QDebug dbg, const QEvent *e)
         const Qt::MouseButtons buttons = spe->buttons();
         dbg << eventClassName(type) << '(';
         QtDebugUtils::formatQEnum(dbg, type);
+        dbg << " ts=" << spe->timestamp();
         if (isMouse) {
             if (type != QEvent::MouseMove && type != QEvent::NonClientAreaMouseMove) {
                 dbg << ' ';