POLAR@3: Add and remove plugin via DOM

[oeninghe-dataport]

We already use custom elements to add and remove our map to the DOM.

It might be an idea to have custom elements per-plugin (e.g., <polar-plugin-fullscreen>), that can be inserted in the inner HTML of a polar-map. This insertion in the DOM then replaces addPlugin (which may exist as a convenience helper such as createMap then).

Managing the plugin's stores via Pinia (src/core/stores/plugin) could be superfluous after this change.