Globally exclusive layers

[silvester-pari]

Currently, exclusive layers only check layers on the same "level" (i.e. in the same group) for exclusivity. There might be usecases where we want an exclusive selection for all exclusive layers in the entire map, e.g. multiple groups of background layers, and only one of those layers can be shown at a time.

The tricky part here will be to deal with the layer group checkboxes, as when no layer is selected inside a group anymore (because an exclusive layer from another group was set to visible), then the group checkbox doesn't make sense.

Maybe we need a way to either automatically set the checkbox to false if all group layers are hidden, or some way to not show the checkbox at all.