Create 'edit_state' object in data to manage, well, state of edits.
In edit mode, currently selected point is draggable.

Furthermore, dragging it alters the stored geoJson, and is correctly
reflected on export, all managed by the edit_state component (which is
listening for "selectFeature" and "selectedFeatureMoved" events).

Works when tested manually, and verifies the general theory of edits for
other stuff (and how I expect edit state to be bound to the UI, at least in
general terms).  Automated tests tbd.