Add legend and/or value for custom external layers

[nesnoj]

In #20 custom layers are introduced.
However, two important features are missing to make those layers useful:

1. Value legend (here I added the legend from the WMS LegendURL to the content when hovering (i) )
2. Click on layer shows value for this point

(Example layer in the BB Geoviewer can be found here)

Maybe helpful