Clang: reduce warnings

- mark some things override
 - make conversion of 0 to flags explicit

src/calamares/CalamaresWindow.h

@@ -37,7 +37,7 @@ class CalamaresWindow : public QWidget
     Q_OBJECT
 public:
     CalamaresWindow( QWidget* parent = nullptr );
-    virtual ~CalamaresWindow() {}
+    virtual ~CalamaresWindow() override {}
 
 public slots:
     /**

src/modules/netinstall/PackageModel.cpp

@@ -144,7 +144,7 @@ Qt::ItemFlags
 PackageModel::flags( const QModelIndex& index ) const
 {
     if ( !index.isValid() )
-        return 0;
+        return Qt::ItemFlags();
     if ( index.column() == 0 )
         return Qt::ItemIsUserCheckable | QAbstractItemModel::flags( index );
     return QAbstractItemModel::flags( index );

src/modules/tracking/TrackingJobs.h

@@ -30,7 +30,7 @@ class TrackingInstallJob : public Calamares::Job
     Q_OBJECT
 public:
     TrackingInstallJob( const QString& url );
-    ~TrackingInstallJob();
+    ~TrackingInstallJob() override;
 
     QString prettyName() const override;
     QString prettyDescription() const override;