Add interface to request the pid of the running instance.

src/qtsingleapp/qtsingleapplication.h

@@ -88,6 +88,9 @@ public:
     // Obsolete:
     void initialize(bool dummy = true)
         { isRunning(); Q_UNUSED(dummy) }
+#ifdef Q_OS_WIN
+    ulong getRunningPid();
+#endif
 
 public Q_SLOTS:
     bool sendMessage(const QString &message, int timeout = 5000);