Ticket #61 (closed feature: fixed)

Description

Copied from #32:

In a broader context, the checkbox-style is great for transparent layers, and the radio-style is better for base layers. Ultimately what we want is a smart LayerSwitcher? that is capable of doing radio-style selection between its base layers, and checkbox-style selection otherwise, *all* in the same switcher.

This will require going through each Layer class and adding some notion of "baseness".