Map responds to external selectedFeatureMove events, if they ever come.

Right now, I'm just being consistent; this will matter if these events
ever come from elsewhere (e.g., updated on changes to address properties).
Barebones edit-state support for generic prop-change events.

Now to integrate a JSON editor so we can start sending them...
Start on the property-editing UI.

Using Jeremy Dorn's json editor, replacing the contents of the info window.
Needs a little glue code to connect up to the edit state object.  More
seriously, needs to have either scrollbars, much smaller fonts, or both,
to get adequate space.