[ench] Slider UI scaling

[theludovyc]

The UI can look tiny in high res :

Today, to fix this lot of games have a slider to increase the UI scale. We can add it bellow the resolution combobox.

I suppose we need to set the scale factor https://docs.godotengine.org/en/stable/classes/class_window.html#class-window-property-content-scale-factor

For more information see https://docs.godotengine.org/en/stable/tutorials/rendering/multiple_resolutions.html#stretch-scale