[Mac] Displaying commands Description as Tooltips on MenuItems in MenuBar

This is basically same as https://github.com/mono/monodevelop/pull/487 but for OSX...
![image](https://cloud.githubusercontent.com/assets/774791/4045235/fbdaccd6-2d27-11e4-83b6-f10641bb8c77.png)

![image](https://cloud.githubusercontent.com/assets/774791/4045248/2cff445e-2d28-11e4-870a-743b41e4a016.png)

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