Moved fullscreen and keep screen on enablers to ACInterface.
Let the os handle volume buttons.
Made acLib cleanly handle not having a current world.
Added load function prototype to acLib.hpp.

So that a compile time error is created if the user creates the function incorrectly.
Removed pointless pointerData pointer in ACInput.
Added multi-touch average distance info to acPointerData.

Mostly used for pinch to zoom type situations.