feat: fit by width

Signed-off-by: Gary Wang <wzc782970009@gmail.com>

app/mainwindow.h

@@ -66,6 +66,7 @@ private slots:
     void on_actionZoomIn_triggered();
     void on_actionZoomOut_triggered();
     void on_actionHorizontalFlip_triggered();
+    void on_actionFitByWidth_triggered();
     void on_actionCopyPixmap_triggered();
     void on_actionCopyFilePath_triggered();
     void on_actionPaste_triggered();