[libcalamares] Give ViewManager a real quit() slot

- instead of creating a lambda, give ViewManager a real slot
- hook up the new navigation buttons to the next, back, quit slots

src/calamares/CalamaresWindow.cpp

@@ -158,10 +158,13 @@ CalamaresWindow::getWidgetNavigation()
     // Create buttons and sets an initial icon; the icons may change
     auto* back = new QPushButton( getButtonIcon( QStringLiteral( "go-previous" ) ), tr( "&Back" ), navigation );
     back->setObjectName( "view-button-back" );
+    connect( back, &QPushButton::clicked, m_viewManager, &Calamares::ViewManager::back );
     auto* next = new QPushButton( getButtonIcon( QStringLiteral( "go-next" ) ), tr( "&Next" ), navigation );
     next->setObjectName( "view-button-next" );
+    connect( next, &QPushButton::clicked, m_viewManager, &Calamares::ViewManager::next );
     auto* quit = new QPushButton( getButtonIcon( QStringLiteral( "dialog-cancel" ) ), tr( "&Cancel" ), navigation );
     quit->setObjectName( "view-button-cancel" );
+    connect( quit, &QPushButton::clicked, m_viewManager, &Calamares::ViewManager::quit );
 
     QBoxLayout* bottomLayout = new QHBoxLayout;
     bottomLayout->addStretch();

src/libcalamaresui/ViewManager.cpp

@@ -108,14 +108,9 @@ ViewManager::ViewManager( QObject* parent )
 
     connect( m_next, &QPushButton::clicked, this, &ViewManager::next );
     connect( m_back, &QPushButton::clicked, this, &ViewManager::back );
+    connect( m_quit, &QPushButton::clicked, this, &ViewManager::quit );
     m_back->setEnabled( false );
 
-    connect( m_quit, &QPushButton::clicked, this, [this]() {
-        if ( this->confirmCancelInstallation() )
-        {
-            qApp->quit();
-        }
-    } );
     connect( JobQueue::instance(), &JobQueue::failed, this, &ViewManager::onInstallationFailed );
     connect( JobQueue::instance(), &JobQueue::finished, this, &ViewManager::next );
 
@@ -476,6 +471,16 @@ ViewManager::back()
     updateButtonLabels();
 }
 
+
+void
+ViewManager::quit()
+{
+    if ( confirmCancelInstallation() )
+    {
+        qApp->quit();
+    }
+}
+
 bool
 ViewManager::confirmCancelInstallation()
 {

src/libcalamaresui/ViewManager.h

@@ -92,7 +92,7 @@ public:
      */
     bool confirmCancelInstallation();
 
-public slots:
+public Q_SLOTS:
     /**
      * @brief next moves forward to the next page of the current ViewStep (if any),
      * or to the first page of the next ViewStep if the current ViewStep doesn't
@@ -107,6 +107,13 @@ public slots:
      */
     void back();
 
+    /**
+     * @brief Probably quit
+     *
+     * Asks for confirmation if necessary. Terminates the application.
+     */
+    void quit();
+
     /**
      * @brief onInstallationFailed displays an error message when a fatal failure
      * happens in a ViewStep.