Wait for dom:loaded to fire before initialising the map

Also very all our use of window.onload and switch to using dom:loaded
instead where we can - the main case where we can't is when using a
vector layer as IE will fail unless the page is completely loaded.

app/views/changeset/_map.html.erb

@@ -57,7 +57,8 @@
     var bbox = new OpenLayers.Bounds(minlon, minlat, maxlon, maxlat);
 
     bounds.extend(bbox);
-    box = addBoxToMap(bbox, "<%= edit.id %>", true);
+
+    addBoxToMap(bbox, "<%= edit.id %>", true);
     <% end %>
 
     vectors.events.on({