clang-tidy: Apply modernize-use-override everywhere (#6439)

... to mark overriding functions `override` instead of `virtual`.

src/core/PresetPreviewPlayHandle.cpp

@@ -53,7 +53,7 @@ public:
 		m_previewInstrumentTrack->setPreviewMode( true );
 	}
 
-	virtual ~PreviewTrackContainer()
+	~PreviewTrackContainer() override
 	{
 	}
 

src/gui/MixerView.cpp

@@ -113,7 +113,7 @@ MixerView::MixerView() :
 		public:
 			ChannelArea( QWidget * parent, MixerView * mv ) :
 				QScrollArea( parent ), m_mv( mv ) {}
-			~ChannelArea() {}
+			~ChannelArea() override {}
 			void keyPressEvent( QKeyEvent * e ) override
 			{
 				m_mv->keyPressEvent( e );

src/gui/RowTableView.cpp

@@ -41,13 +41,13 @@ public:
 		m_table( table )
 		{
 		}
-	virtual void paint( QPainter * painter,
+	void paint( QPainter * painter,
 					const QStyleOptionViewItem & option,
 					const QModelIndex & index ) const override;
 
 
 protected:
-	virtual void initStyleOption( QStyleOptionViewItem * option,
+	void initStyleOption( QStyleOptionViewItem * option,
 					const QModelIndex & index ) const override;
 
 

src/gui/SideBar.cpp

@@ -44,7 +44,7 @@ public:
 	{
 	}
 
-	virtual ~SideBarButton() = default;
+	~SideBarButton() override = default;
 
 	Qt::Orientation orientation() const
 	{

src/gui/modals/ControllerConnectionDialog.cpp

@@ -60,7 +60,7 @@ public:
 	}
 
 
-	virtual ~AutoDetectMidiController()
+	~AutoDetectMidiController() override
 	{
 	}