Ios: Only ever show a single dialog for developer mode

It can happen that this is triggered twice for the same device, or for
other devices when the dialog is still open. This looks funny and is not
necessary, so avoid it.

Task-number: QTBUG-121557
Change-Id: I0329104b3825b68b565ca1f8e00d785952c9d767
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>

src/plugins/ios/iosdevice.h

@@ -8,6 +8,7 @@
 #include <projectexplorer/devicesupport/idevice.h>
 #include <projectexplorer/devicesupport/idevicefactory.h>
 
+#include <QMessageBox>
 #include <QTimer>
 
 namespace Ios {
@@ -83,6 +84,7 @@ private:
     IosDeviceManager(QObject *parent = nullptr);
     QTimer m_userModeDevicesTimer;
     QStringList m_userModeDeviceIds;
+    QPointer<QMessageBox> m_devModeDialog;
 };
 
 } // namespace Internal