Added Advanced > UI > Show FPS

Added a menu toggle item at Advanced > UI > Show FPS to show fps as a ui element at the bottom right corner.
The related Debug Setting is: SLBShowFPS

![screenshot from 2013-09-22 01 10 11](https://f.cloud.github.com/assets/1002978/1186672/2bddf4fc-2316-11e3-98bf-0dcd84ef50ab.png)