System should use miliseconds instead of frames for clocks.

[RobertWHurst]

Currently the engines clocks are based on the passage of frames. This is unreliable on slow devices and makes audio syncing impossible. The whole engine must be converted to use milliseconds instead of frames.

[RobertWHurst]

Frames will still be used, but a new very accurate frame-skip system has been added. Clocks and timers should now correspond with the expected fps regardless of actual frame rate.