restore collapsed state for cells

js refactor removed this, but did not document the change.  This restores it, but it would be fine to revert this and update the docs instead.

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

@@ -22,6 +22,7 b' var IPython = (function (IPython) {'
         this.code_mirror = null;
         this.input_prompt_number = null;
         this.tooltip_on_tab = true;
+        this.collapsed = false;
         IPython.Cell.apply(this, arguments);
     };
 
@@ -200,17 +201,20 b' var IPython = (function (IPython) {'
 
 
     CodeCell.prototype.collapse = function () {
-    this.output_area.collapse();
+        this.collapsed = true;
+        this.output_area.collapse();
     };
 
 
     CodeCell.prototype.expand = function () {
-    this.output_area.expand();
+        this.collapsed = false;
+        this.output_area.expand();
     };
 
 
     CodeCell.prototype.toggle_output = function () {
-    this.output_area.toggle_output();
+        this.collapsed = Boolean(1 - this.collapsed);
+        this.output_area.toggle_output();
     };