Emulate "Switch background map"

[jidanni]

I'm just thinking, in order to emulate the "Switch background map" function of other apps and programs, maybe Viking, in addition to having commands to add and remove map layers, could also have a combined function, that does both tasks at once: adds the new, and removes the old, from the layer stack. No point in keeping a layer that the user no matter wants. (But yes, do keep the tiles downloaded.)