Background layer in catalog tree

Apparently, we can have layers with the background attribute set to true in the catalog tree (I think that's a reasonable use case). This results in a bug related to catalog tree:

If `Color Map` is set as background, then the following happens:
- On Hover, Color Map is not shown as Preview -> it should be shown as preview
- On Click, Background Layer gets removed from map -> it should be added on top without touching the background layer. This means to add a second instance of the same layer to the map.

@davidoesch Can you verify that my `should` is correct?