RFE: Add the `disabled` css class for out-of-zoom-range layer groups as well

[cwassert]

Since a LayerGroup can also have a minResolution, maxResolution, minZoom and maxZoom, it makes sense to add this feature to layer groups as well.

[walkermatt]

Hi, Currently it's assumed that a layer has either min/max resolution or min/max zoom. I've not considered anyone using both on a single layer. It should be quite straightforward to update the current logic to take both into account. If you would like to implement the change I'd be happy to review a pull request.

[gmerciel]

I think @cwassert is referring to the fact that LayerGroups are not getting the disabled class when not shown because of its zoom or resolution, It has nothing to do with using both configurations (resolution and zoom).