Add click event as responsive alternative to singleclick

The singleclick event makes the application feel slow because
of the 250 ms delay that is used to rule out a dblclick.

examples/image-vector-layer.js

@@ -86,6 +86,6 @@ $(map.getViewport()).on('mousemove', function(evt) {
   displayFeatureInfo(pixel);
 });
 
-map.on('singleclick', function(evt) {
+map.on('click', function(evt) {
   displayFeatureInfo(evt.pixel);
 });