Doc: Fix QThread documentation snippet

WorkerThread needs an explicit constructor, otherwise

    WorkerThread *workerThread = new WorkerThread(this);

will fail. While at it, at some infrastructure code so that the
.cpp file actually compiles.

Change-Id: I35835b1be3da2a1f75222594197f6013b41a7b4a
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
(cherry picked from commit 68a16558171abccd15f5446ac208e105cf337dc0)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/doc/snippets/code/src_corelib_thread_qthread.cpp

@@ -2,12 +2,21 @@
 // SPDX-License-Identifier: LicenseRef-Qt-Commercial OR BSD-3-Clause
 
 #include <QtCore/QThread>
-class MyObject;
+class MyObject : public QObject
+{
+    Q_OBJECT
+    void startWorkInAThread();
+private slots:
+    void handleResults(){};
+};
 
 //! [reimpl-run]
 class WorkerThread : public QThread
 {
     Q_OBJECT
+public:
+    explicit WorkerThread(QObject *parent = nullptr) : QThread(parent) { }
+protected:
     void run() override {
         QString result;
         /* ... here is the expensive or blocking operation ... */
@@ -66,3 +75,7 @@ signals:
     void operate(const QString &);
 };
 //! [worker]
+
+void Controller::handleResults(const QString &) { }
+
+#include "src_corelib_thread_qthread.moc"