revert: remove auto long image mode

- remove auto long image mode.
- there is no "auto" requirement for this.
- user can utilize fit by height/width to achieve the same goal by manually click them.
- some optimization code about graphic view are preserved.

app/mainwindow.h

@@ -93,7 +93,6 @@ private slots:
     void on_actionHorizontalFlip_triggered();
     void on_actionFitInView_triggered();
     void on_actionFitByWidth_triggered();
-    void on_actionFitLongImage_triggered();
     void on_actionCopyPixmap_triggered();
     void on_actionCopyFilePath_triggered();
     void on_actionPaste_triggered();