Support for requestAnimationFrame(callback) is missing

Next to the scheduling functions `setInterval(function, delay)` and `setTimeout(function, delay)` Tabris.js should also support the third variant `requestAnimationFrame(callback)`.

See [Schedule Update in Animations](https://developer.mozilla.org/en-US/docs/Web/API/Canvas_API/Tutorial/Basic_animations#Scheduled_updates) in MDN.

This third variant is useful for animating things.

Currently setTimeout() shows a wide variety in number of executions per second.
- If you alter a native widget in the callback, number of executions is capped by vsynch (60 calls per scond).
- If you draw on a canvas, you get around 100 calls per second.
- If you just increment a counter without altering the UI, you get around 4000 calls per second.