Support for legends

[fschmenger]

There should be some generic support to display legend images in Wegue, so we can have useful annotations for layers.
An example screenshot taken from the geoserver docs:

I open this issue as a point for discussion, where those legends should be placed, what the scope of legends should be and how a structured implementation may look like.

For a first start, we could associate legends with layers and enhance the mapLayers app-conf definition, e.g.

"mapLayers": [
    {
      "lid": "my-layer",
      "legendUrl": "http://somewhere",
      ....
    }
]

Then we could display the legend images within the LayerList component.

[chrismayer]

FYI: There is a working project solution for a legend module by @JakobMiksch.