Replaced NSTimer with CADisplayLink.

This PR implements part of the discussion that started in https://github.com/mono/MonoGame/pull/1030 but severely got off topic.

Apple's documentation states that an NSTimer's effective resolution is limited to 50-100ms. Using a CADisplayLink is a preferred way to implement a high resolution timer, as we can see resolutions as fast as ~16-17ms.

Depending on the project there may or may not be a performance gain from this.