Add example with simple permalink support

This PR is still work in progress. For some reason the panning does not work (mouse cursor is always "finger" when the map is loaded)!?
Since this example is an evolution of the "map-extension" example, it might also be transfered in it.
When the URL params are updated by hand, the whole application is reloaded. I found it consistent to place the params detection at the same place than the code that manage the creation of the map (with its initial state).

But OK to have the URL-params reading/writing code at the same place.