Revamp Queued Custom Type Ex: Declare stopProcess() as a method

Declare stopProcess() as a method because it's used in the connect()
overload taking a pointer-to-member-function, so it doesn't need help
from moc.

Task-number: QTBUG-117147
Pick-to: 6.5
Change-Id: Iaa1c02de82cffdea455af520c37dc0e6e6861443
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>
(cherry picked from commit 74318643ced197f9380d21c3dfc662a85e73f5b8)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

examples/corelib/threads/queuedcustomtype/renderthread.h

@@ -20,13 +20,11 @@ public:
     ~RenderThread();
 
     void processImage(const QImage &image);
+    void stopProcess();
 
 signals:
     void sendBlock(const Block &block);
 
-public slots:
-    void stopProcess();
-
 protected:
     void run();