revert: revert feature about saving image as new file with different format.

This reverts commit 5b2255e903.

The feature about saving image as new file with different format is not the goal of an image viewer.
There is no need to keep feature diversity in this fork.
So I simply remove it.

app/mainwindow.h

@@ -75,7 +75,6 @@ protected:
 
 private slots:
     void on_actionOpen_triggered();
-    void on_actionSaveAs_triggered();
 
     void on_actionActualSize_triggered();
     void on_actionToggleMaximize_triggered();