Display Message to explain Gold Foil not used

When the user selects the gold foil measurement mode, a message appears
to explain that this part is currently not used.

src/clientgui.cpp

@@ -59,13 +59,15 @@ ClientGui::ClientGui(const QString& name, const QString& token, std::unique_ptr<
     connect(_ui.cmdCloseVacuumValve, &QPushButton::clicked, std::bind(&ClientGui::sendSimpleActiontoServer, this, Protocol::Action::closeVacuumValve));
     connect(_ui.cmdTargetClose, &QPushButton::clicked, std::bind(&ClientGui::setBeamholeState, this, ExperimentState::BeamHoleState::Closed));
     connect(_ui.cmdTargetOpen, &QPushButton::clicked, std::bind(&ClientGui::setBeamholeState, this, ExperimentState::BeamHoleState::Open));
-    connect(_ui.cmdTargetGold, &QPushButton::clicked, std::bind(&ClientGui::setBeamholeState, this, ExperimentState::BeamHoleState::GoldFoil));
+    //connect(_ui.cmdTargetGold, &QPushButton::clicked, std::bind(&ClientGui::setBeamholeState, this, ExperimentState::BeamHoleState::GoldFoil));
     connect(_ui.cmdStartAdc, &QPushButton::clicked, std::bind(&ClientGui::sendSimpleActiontoServer, this, Protocol::Action::startAdc));
     connect(_ui.cmdStopAdc, &QPushButton::clicked, std::bind(&ClientGui::sendSimpleActiontoServer, this, Protocol::Action::stopAdc));
     connect(_ui.cmdClearHisto, &QPushButton::clicked, this, &ClientGui::clearHistogram);
     connect(_ui.cmdAdcThresholdUp, &QPushButton::clicked, std::bind(&ClientGui::sendSimpleActiontoServer, this, Protocol::Action::adcThresholdUp));
     connect(_ui.cmdAdcThresholdDown, &QPushButton::clicked, std::bind(&ClientGui::sendSimpleActiontoServer, this, Protocol::Action::adcThresholdDown));
 
+    connect(_ui.cmdTargetGold, &QPushButton::clicked, [this] () -> void {QMessageBox::information(this, tr("Experiment Part not available"), tr("This part of the experiment is currently left out, because it has also been left out in the real experiment in the recent years.\nThis measurement only confirms the calibraton of the setup and takes a long time due to the low activity of the real-world ²⁴¹Am source."), QMessageBox::Ok);});
+
     gStyle->SetCanvasPreferGL(true);
 
     connect(&_rootTimer, &QTimer::timeout, []() -> void {gSystem->ProcessEvents();});