Ensure that WMS layer groups of type "Named Tree" are supported

[jacobwod]

This was originally a comment to issue #197

Proposal: ensure that WMS layer groups of type "Named Tree" are listed.

As of today, one must select "Single" or "Opaque Container" to make the layer group show up in Hajk's admin. This makes it impossible to use those other modes, especially "Named Tree", that lists not only the layer group but also the sub-layers inside the group. It works great in QGIS, and it would be really useful if we could get the same functionality in Hajk.

QGIS:

GeoServer:

Below is an example of two layer groups. topowebb-color is configured with mode "Opaque Container", topowebb-light has mode "Named Tree". A layer group configured in as "Named Tree" does not show up at all in Hajk's admin, even though it's there in the response from GetCapabilities:

I'm not really sure why Hajk works like this today, as it seems to ignore any <Layer> nodes that have nested <Layer>s inside. That's the only difference I can see – other than that the nodes look identical.

Originally posted by @jacobwod in https://github.com/hajkmap/Hajk/issues/197#issuecomment-476671460

[jacobwod]

Proposal: ensure that WMS layer groups of type "Named Tree" are listed.

A temporary solution has been added as of be1d9feb, looks something like this: