Re-design what mapml-viewer does with LegendURL / GetLegendGraphic links

[prushforth]

WMS and other OGC mapping services provide legend information via linked graphics, typically but not necessarily images.

In the original implementation of this custom element, I wasn't sure what to do with this information, but I wanted to expose it, so I included a <a href="LegendURL">Layer name/title/label</a> in the <details>' <summary> that represents the layer in the layer control.

It seems possible that browsers will in future disallow such interactive content in the <summary> elements, so this issue is where we can discuss and design what to do with the (set of possible) LegendURL or GetLegendGraphic links (or equivalents) that may be available for a map layer. Currently these can be represented by DOM elements such as

<map-link rel="legend" href="LegendURL"></map-link>

but the question remains as to how to expose them to the developer, if not the user, in an accessible manner.