Menu actions

Draft/Proof of concept. Done in the car while moving from one flat to another (no I was not driving).

So what are you suppose to see here. 
Purely declarative menu form: 
```
"Edit",[
                'ipython.cut-selected-cell',
                'ipython.copy-selected-cell',
                'ipython.paste-cell-before',
                'ipython.paste-cell-after',
                '<divider>',
                'ipython.delete-cell',
                '<divider>',
                ['Submenu',[
                    'Action 1',
                    'Action 2',
                    'Action 3'
                    ]
                ]
```

And automatically show icon/shortcut if associated with action. 

![capture d ecran 2014-12-22 a 18 29 49](https://cloud.githubusercontent.com/assets/335567/5528477/0bf93fce-8a09-11e4-9282-382492f16929.png)

Still highly work in progress.