Add event/callbacks for forced pausing

[Varriount]

In systems such as iOS, Android, Wii U, etc. applications can be "paused" by the user (by exciting the app, pressing the home button, etc).Support to intercept these events is quite important, as often the GPU must be reset and primed when switching back.

Thus, I propose the addition of pause/unpause events/callbacks.

[zacharycarter]

This needs a lot of investigation and a lot of changes are needed in the core of FRAG to properly support android.

I'm going to need either -
A) Someone who is interested in trying to build a mobile game with FRAG who can work closely with me on implementing what needs to be implemented
B) Some time to do the above myself

Either way this is high priority and should be tackled soon.