Add an infoText when grouping pixel/bank

In the Draw tab of the instrument viewer, when grouping by pixel or
bank, the infotext at the bottom informs the user of what is selected,
so there is at least some info on what is going on.

qt/widgets/instrumentview/inc/MantidQtWidgets/InstrumentView/InstrumentWidget.h

@@ -229,7 +229,7 @@ public slots:
   /// Enable OpenGL. Slot called from render tab only - doesn't update the
   /// checkbox.
   void enableGL(bool on);
-  void updateInfoText();
+  void updateInfoText(const QString &text = QString());
 
 private slots:
   void helpClicked();

qt/widgets/instrumentview/src/InstrumentWidget.cpp

@@ -422,7 +422,13 @@ QString InstrumentWidget::getSaveFileName(const QString &title,
 /**
  * Update the info text displayed at the bottom of the window.
  */
-void InstrumentWidget::updateInfoText() { setInfoText(getSurfaceInfoText()); }
+void InstrumentWidget::updateInfoText(const QString &text) {
+  if (text.length() == 0) {
+    setInfoText(getSurfaceInfoText());
+  } else {
+    setInfoText(text);
+  }
+}
 
 void InstrumentWidget::setSurfaceType(int type) {
   // we cannot do 3D without OpenGL

qt/widgets/instrumentview/src/InstrumentWidgetMaskTab.cpp

@@ -81,7 +81,6 @@ InstrumentWidgetMaskTab::InstrumentWidgetMaskTab(InstrumentWidget *instrWidget)
       m_doubleManager(nullptr), m_browser(nullptr), m_left(nullptr),
       m_top(nullptr), m_right(nullptr), m_bottom(nullptr) {
 
-  m_detectorsToGroup = QList<Mantid::detid_t>();
   // main layout
   QVBoxLayout *layout = new QVBoxLayout(this);
 
@@ -226,7 +225,7 @@ InstrumentWidgetMaskTab::InstrumentWidgetMaskTab(InstrumentWidget *instrWidget)
 
   // Algorithm buttons
 
-  m_applyToData = new QPushButton("Apply to Data");
+  m_applyToData = new QPushButton("Apply to Data (Cannot be reverted)");
   m_applyToData->setToolTip("Apply current detector and bin masks to the data "
                             "workspace. Cannot be reverted.");
   connect(m_applyToData, SIGNAL(clicked()), this, SLOT(applyMask()));
@@ -456,7 +455,6 @@ void InstrumentWidgetMaskTab::selectTool(Activity tool) {
 void InstrumentWidgetMaskTab::setActivity() {
   const QColor borderColor = getShapeBorderColor();
   const QColor fillColor = getShapeFillColor();
-  // m_detectorsToGroup.clear();
   QString whatIsBeingSelected = m_maskBins && getMode() == Mode::Mask
                                     ? "Selecting bins"
                                     : "Selecting detectors";
@@ -524,7 +522,6 @@ void InstrumentWidgetMaskTab::setActivity() {
     m_activeTool->setText("Tool: Tube/bank mask. " + whatIsBeingSelected);
   }
   m_instrWidget->updateInfoText();
-  // enableApplyButtons();
 }
 
 /**
@@ -538,6 +535,8 @@ void InstrumentWidgetMaskTab::singlePixelPicked(size_t pickID) {
     return;
   }
 
+  size_t parent;
+
   QApplication::setOverrideCursor(QCursor(Qt::WaitCursor));
   std::vector<size_t> detectorsId{pickID};
   m_instrWidget->updateInstrumentView(); // to refresh the pick image
@@ -547,7 +546,7 @@ void InstrumentWidgetMaskTab::singlePixelPicked(size_t pickID) {
       if (!componentInfo.hasParent(pickID)) {
         return;
       }
-      auto parent = componentInfo.parent(pickID);
+      parent = componentInfo.parent(pickID);
       detectorsId = componentInfo.detectorsInSubtree(parent);
     }
     storeDetectorMask(m_roi_on->isChecked(), detectorsId);
@@ -561,7 +560,7 @@ void InstrumentWidgetMaskTab::singlePixelPicked(size_t pickID) {
       if (!componentInfo.hasParent(pickID)) {
         return;
       }
-      const auto parent = componentInfo.parent(pickID);
+      parent = componentInfo.parent(pickID);
       const auto dets =
           actor.getDetIDs(componentInfo.detectorsInSubtree(parent));
       m_detectorsToGroup.clear();
@@ -575,6 +574,19 @@ void InstrumentWidgetMaskTab::singlePixelPicked(size_t pickID) {
   m_instrWidget->updateInstrumentDetectors();
   QApplication::restoreOverrideCursor();
   enableApplyButtons();
+
+  if (m_grouping_on->isChecked()) {
+    if (m_pixel->isChecked()) {
+      m_instrWidget->updateInfoText(
+          QString("Pixel %0 picked for grouping").arg(pickID));
+
+    } else if (m_tube->isChecked()) {
+      std::string message = "Parent " +
+                            componentInfo.componentID(parent)->getName() +
+                            " picked for grouping";
+      m_instrWidget->updateInfoText(QString::fromStdString(message));
+    }
+  }
 }
 
 /**