fix sticky warning.

Widget.warning('text....') was infinitely sticky in some cases
like content manager raising 'not implemented' on copy.

The make default click_callback to dismiss notification,
and fix the logic to not unbind the click handler if it requests
the notification not to be dismissed.

IPython/html/static/base/js/notificationwidget.js

@@ -99,19 +99,22 b' define(['
             }, timeout);
         }
 
-        // bind the click callback if it is given
-        if (click_callback !== undefined) {
+        // if no click callback assume we will just dismiss the notification
+        if (click_callback === undefined) {
+            click_callback = function(){return true};
+        }
+        // on click, remove widget if click callback say so
+        // and unbind click event.
             this.element.click(function () {
                 if (click_callback() !== false) {
                     that.element.fadeOut(100, function () {that.inner.text('');});
-                }
                 that.element.unbind('click');
+            }
                 if (that.timeout !== null) {
                     clearTimeout(that.timeout);
                     that.timeout = null;
                 }
             });
-        }
     };
 
     /**