feat: enable automatic style creation for wms layers' getLegendGraphic

[Grammostola]

Proposes to fix #1560 . Introduces two properties of a WMS layer:

      "hasThemeLegend": true,
      "legendParams" : {
        "scale" : 5000,
        "legend_options" : "dpi:300"
      }

Where hasThemeLegend is necessary for "auto" style creation for WMS layers with a theme legend (to extendedLegend) and
legendParams is more or less what @steff-o suggested in the related issue.

Both properties are ignored if a style property also exists for the given WMS layer.

The default "scale" for the getLegendGraphic call (there needs to be a scale for scale dependant styles) is the largest scale of the configured map (because typically theme layers have more visible classes at a larger scale than at a smaller scale if scale dependant) however that value is overriden if scale is given as a legendParam.

The possibility exists to make hasThemeLegend part of the legendParams object but it's slightly easier to merely define the hasThemeLegend property rather than it as part of an object if no special legendParams as per getLegendGraphic() are to be employed.

[jokd]

Tested and LGTM

[Grammostola]

Currently this pr sets the styleName property of a layer while its style property remains 'default'. In comparison the same layer with a style given in index.json will have both the style and styleName property as the name of that style. (layer.js does
layerOptions.styleName = layerOptions.style;)
At a glance I don't see any layer.get('style') references in the origo client, merely layer.get('styleName') so I don't know whether this makes a difference however I can see the logic in making these auto-default-WMS-layers identical to entering a style manually for them. Any thoughts on this?

[tonnyandersson]

@Grammostola Quagmire.

[jokd]

I guess we can live with that

[Grammostola]

Answers clear as glass
Understanding rolling in
So shall it be now

🦦