add menu item for undo delete cell

closes  #2920

IPython/frontend/html/notebook/static/js/menubar.js

@@ -83,6 +83,9 b' var IPython = (function (IPython) {'
         this.element.find('#delete_cell').click(function () {
             IPython.notebook.delete_cell();
         });
+        this.element.find('#undelete_cell').click(function () {
+            IPython.notebook.undelete();
+        });
         this.element.find('#split_cell').click(function () {
             IPython.notebook.split_cell();
         });

IPython/frontend/html/notebook/templates/notebook.html

@@ -77,7 +77,8 b' class="notebook_app"'
                 <li id="paste_cell_above" class="ui-state-disabled"><a href="#">Paste Cell Above</a></li>
                 <li id="paste_cell_below" class="ui-state-disabled"><a href="#">Paste Cell Below</a></li>
                 <li id="paste_cell_replace" class="ui-state-disabled"><a href="#">Paste Cell &amp; Replace</a></li>
-                <li id="delete_cell"><a href="#">Delete</a></li>
+                <li id="delete_cell"><a href="#">Delete Cell</a></li>
+                <li id="undelete_cell"><a href="#">Undo Delete Cell</a></li>
                 <hr/>
                 <li id="split_cell"><a href="#">Split Cell</a></li>
                 <li id="merge_cell_above"><a href="#">Merge Cell Above</a></li>