[calamares] Use a "real" slot for UniqueConnection

Multiple reports of the following fatal error:
    WARNING (Qt): QObject::connect(QItemSelectionModel, PartitionPage):
    unique connections require a pointer to member function of a QObject subclass
suggest to replace the unique lambda by a "real" slot.

src/calamares/progresstree/ProgressTreeView.cpp

@@ -47,10 +47,15 @@ ProgressTreeView::setModel( QAbstractItemModel* model )
 
     QListView::setModel( model );
 
-    connect(
-        Calamares::ViewManager::instance(),
-        &Calamares::ViewManager::currentStepChanged,
-        this,
-        [ this ]() { viewport()->update(); },
-        Qt::UniqueConnection );
+    connect( Calamares::ViewManager::instance(),
+             &Calamares::ViewManager::currentStepChanged,
+             this,
+             &ProgressTreeView::update,
+             Qt::UniqueConnection );
+}
+
+void
+ProgressTreeView::update()
+{
+    viewport()->update();
 }

src/calamares/progresstree/ProgressTreeView.h

@@ -31,6 +31,9 @@ public:
      * @brief setModel assigns a model to this view.
      */
     void setModel( QAbstractItemModel* model ) override;
+
+public Q_SLOTS:
+    void update();
 };
 
 #endif  // PROGRESSTREEVIEW_H