Readded url hash updating

as a "side-effect", the current coords remain centered when switching
tilesets.  at least in theory.  needs more work

overviewer_core/data/js_src/views.js

@@ -209,6 +209,9 @@ overviewer.views.GoogleMapView = Backbone.View.extend({
             }
         }
 
+        // for this world, remember our current viewport (as worldcoords, not LatLng)
+        //
+
     }
 
 });