Show keybinding in main menu.

Show keybinding in main menu.
But the problem is unfortunately I am bad in CSS. so I finally failed to make it right aligned. 

Fixes https://github.com/dart-lang/spark/issues/819
Review @ussuri

TEST= Manual only in Windows.
![image](https://f.cloud.github.com/assets/756988/2120313/528424b4-91ac-11e3-894a-682fc234a00e.png)