Add an event that fires before the notebook saves

IPython/html/static/notebook/js/notebook.js

@@ -1980,6 +1980,10 b' define(['
             return;
         }
 
+        // Trigger an event before save, which allows listeners to modify
+        // the notebook as needed.
+        this.events.trigger('before_save.Notebook');
+
         // Create a JSON model to be sent to the server.
         var model = {
             type : "notebook",

IPython/html/static/widgets/js/manager.js

@@ -37,7 +37,7 b' define(['
 
         // Setup state saving code.
         var that = this;
-        this.notebook.events.on('notebook_saved.Notebook', function() {
+        this.notebook.events.on('before_save.Notebook', function() {
             var save_callback = WidgetManager._save_callback;
             var options = WidgetManager._get_state_options;
             if (save_callback) {