Lazily copied inheritances

[IvanSanchez]

In order to save room, inheritances should be able to be lazily copied within the DOM via JS whenever the user expands them.

This might not be the preferred solution (docs should work without JS). Maybe this can be done via clever templates (output only names of inherited stuff, let the main HTML template have the JS to lazily load everything).

[mourner]

docs should work without JS

I would not be concerned here. Without JS, we can just gracefully degrade to linking the clickable inheritance headings to the original classes instead of expanding. Or just do nothing at all since docs would still be usable without JS (a user sees that a classes inherits Evented and then just goes on to check that class separately).

[mourner]

Let's prioritize this! The Leaflet 1.0 docs take an unreasonable time to load.