webcomponent-bundle.js clones custom elements, prevents actual custom elements from working

[prushforth]

          > I noticed that when rendering features via the extension (if you put a build of this branch in the extension), you can't visually delete features that are [extension-rendered](https://maps4html.org/experiments/shared/restaurants/restaurants.xml) from the layer-.shadowRoot (within the console) i.e. it doesn't remove the linked \<g\> element when you do that.

I believe the "link" / "connection" between the map-feature element and <g> element is set up properly when the extension renders the map, so the codes should work as our expectation in this process:

I think the issue might be related to the webcomponent-bundle.js file. It seems that when we use the extension to render XML files, it may "clone" the custom elements we defined (layer-, map-feature, etc.) and attach the cloned element to the DOM instead of the ones that we create. If you remove the <layer- > element in the experiment (https://maps4html.org/experiments/shared/restaurants/restaurants.xml), you will notice that the layer is still present, just as we observed with <map-feature>. These "cloned" elements in the DOM do not have accessibility for any properties that we set (e.g. <layer- >._layer):

So even though the links between and feature elements that we created are properly set, they are not "reflected" to the "cloned" elements that we can handle in DOM.

Originally posted by @yhy0217 in Maps4HTML/MapML.js#801 (comment)