Implemented Undo Redo Shortcut

Changes
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1. Added Undo shortcut to toolbar
2. Added Redo shortcut to toolbar
3. Fix a bug that does not allow command to disable shortcut
4. Raised CanExecuteChanged for undo and redo

![image](https://f.cloud.github.com/assets/4847382/1788959/d5fde314-6945-11e3-8f7b-155823cd9288.png)