Only include queryable child layers

[tsauerwein]

Closes #962

Please review. I am not sure if this PR is complete, it seems that the format for themes.js has changed. E.g. the themes.js file used in the tests uses this format:

{
  "layers": "hotel",
  "name": "hotel",
  "queryable": 1,
  "childLayers": [],
  ...

... while the server returns this format:

{
  "layers": "hotel",
  "name": "hotel",
  "childLayers": [
    {
      "name": "hotel",
      "queryable": false
    }
  ],
  ...
}

Should both be supported?

[sbrunner]

it seems that the format for themes.js has changed.

Yes it is.

Should both be supported?

No, only the second

[tsauerwein]

Ok, I updated the themes.js file in the tests accordingly. Ready to review.

[sbrunner]

Looks good, thanks @tsauerwein