QT backend bug

Problem: New y-axis max value set by edit-curve interface is forgotten after zoom-in-zoom-out cycle.

Reproduction steps:

* change y-axis max to a larger value than used by the default layouter with the edit-curve interface (click on the green hook)
* Click Ok
* Zoom into the plot
* click the 'Back' button and see the max on y-axis going back to the default value.

I would understand if the Home button goes to the default layout, although that's debatable, but the back button really should go to the previous layout, not the default layout values.