Allow rapidly toggling between background maps (fast restacking of map layers)

[jidanni]

In the manual we see a

Toggle Inverts the visibility status of each item.

That got me thinking. Let's say there are two map layers, A and B. And we want to switch rapidly back and forth between them.

So a keyboard shortcut should be invented.

It could toggle the order of map layers in the stack: top moves down to number 2, which moves up to top. (Or maybe cycle through all map layers, 1, 2, 3, 1, 2 ... But that should have a different keyboard shortcut.)

Or it could simply toggle the visibility of the top layer.