throttle hover for less epileptic experience and better perf

Otherwise the sidebar flashes so fast that it makes no sense while
distracting the user and lagging due to heavy sidebar construction.

js/id/ui/sidebar.js

@@ -32,6 +32,8 @@ iD.ui.Sidebar = function(context) {
             }
         };
 
+        sidebar.hover = _.throttle(sidebar.hover, 200);
+
         sidebar.select = function(id, newFeature) {
             if (!current && id) {
                 featureListWrap.classed('inspector-hidden', true);