Add a simple 'undo' for cell deletion.

I've accidentally deleted the wrong cell quite a few times and
it has been quite frustrating. This enables a very hacky backup of
the last cell deletion.

I'm sure the core devs will have some opinions on both the feature
and its implementation, I consider this very much a first pass.

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

@@ -22,6 +22,9 b' var IPython = (function (IPython) {'
         this.next_prompt_number = 1;
         this.kernel = null;
         this.clipboard = null;
+        this.undelete_backup = null;
+        this.undelete_index = null;
+        this.undelete_below = false;
         this.paste_enabled = false;
         this.dirty = false;
         this.metadata = {};
@@ -257,6 +260,10 b' var IPython = (function (IPython) {'
                 IPython.quick_help.show_keyboard_shortcuts();
                 that.control_key_active = false;
                 return false;
+            } else if (event.which === 90 && that.control_key_active) {
+                that.undelete();
+                that.control_key_active = false;
+                return false;
             } else if (that.control_key_active) {
                 that.control_key_active = false;
                 return true;
@@ -536,13 +543,19 b' var IPython = (function (IPython) {'
 
     Notebook.prototype.delete_cell = function (index) {
         var i = this.index_or_selected(index);
+        var cell = this.get_selected_cell();
+        this.undelete_backup = cell.toJSON();
         if (this.is_valid_cell_index(i)) {
             var ce = this.get_cell_element(i);
             ce.remove();
             if (i === (this.ncells())) {
                 this.select(i-1);
+                this.undelete_index = i - 1;
+                this.undelete_below = true;
             } else {
                 this.select(i);
+                this.undelete_index = i;
+                this.undelete_below = false;
             };
             this.dirty = true;
         };
@@ -818,6 +831,33 b' var IPython = (function (IPython) {'
         };
     };
 
+    // Cell undelete
+
+    Notebook.prototype.undelete = function() {
+        if (this.undelete_backup !== null && this.undelete_index !== null) {
+            var current_index = this.get_selected_index();
+            if (this.undelete_index < current_index) {
+                current_index = current_index + 1;
+            }
+            if (this.undelete_index >= this.ncells()) {
+                this.select(this.ncells() - 1);
+            }
+            else {
+                this.select(this.undelete_index);
+            }
+            var cell_data = this.undelete_backup;
+            var new_cell = null;
+            if (this.undelete_below) {
+                new_cell = this.insert_cell_below(cell_data.cell_type);
+            } else {
+                new_cell = this.insert_cell_above(cell_data.cell_type);
+            }
+            new_cell.fromJSON(cell_data);
+            this.select(current_index);
+            this.undelete_backup = null;
+            this.undelete_index = null;
+        }
+    }
 
     // Split/merge
 

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

@@ -49,7 +49,8 b' var IPython = (function (IPython) {'
             {key: 'Ctrl-m n', help: 'select next'},
             {key: 'Ctrl-m i', help: 'interrupt kernel'},
             {key: 'Ctrl-m .', help: 'restart kernel'},
-            {key: 'Ctrl-m h', help: 'show keyboard shortcuts'}
+            {key: 'Ctrl-m h', help: 'show keyboard shortcuts'},
+            {key: 'Ctrl-m z', help: 'undo last cell deletion'}
         ];
         for (var i=0; i<shortcuts.length; i++) {
             dialog.append($('<div>').