feat: add "Keep transformation" to menu

This could be helpful when viewing a gallery with similar size, which
need to apply same transformation to the view. E.g. When viewing a batch
of pixel image sprite assets, the view might need to be zoomed to clearly
view the detail of the image.

Other use-case can be seen at the linked issue below.

https://github.com/BLumia/pineapple-pictures/issues/87
https://github.com/BLumia/pineapple-pictures/issues/89

app/aboutdialog.cpp

@@ -41,6 +41,9 @@ AboutDialog::AboutDialog(QWidget *parent)
         QStringLiteral("<li><b>%1</b>:<br/>%2</li>")
                 .arg(QCoreApplication::translate("MainWindow", "Protected mode"))
                 .arg(tr("Avoid close window accidentally. (eg. by double clicking the window)")),
+        QStringLiteral("<li><b>%1</b>:<br/>%2</li>")
+                .arg(QCoreApplication::translate("MainWindow", "Keep transformation"))
+                .arg(tr("Avoid resetting the zoom/rotation/flip state that was applied to the image view when switching between images.")),
         QStringLiteral("</ul>")
     };