Selecting "Spanning screens" does not disable configuration dialog on the 2nd/3rd/etc display

Selecting "Spanning screens", which is (correctly) available only on the 1st display, does not disable display settings on the other displays so the user can get confused why desktop background changes made via the settings dialog on the 2nd/3rd display have no effect if "Spanning screens" is enabled.

Steps to reproduce:

1. Set up a virtual machine (e.g: Xubuntu with Xfce 4.16 in VirtualBox) with 2 or more monitors
2. Open "Desktop settings" dialog on the 1st monitor and select "Spanning screens" from the "Style:" menu
3. Open "Desktop settings" dialog on the 2nd monitor: changing the settings has no effect

Suggested solution: Disable the background settings dialog on the 2nd/3rd/etc screens if the "Spanning screens" style is activated.

Screenshot: