Moved parts in Universal_System and Audio to enigma_user.

The changes have been verified to work under Linux and
Windows. The only exception is the Android audio system,
which has not been tested.

There are still some parts in Universal_System that have
not been moved, but some of those parts will be easier
to move once Graphics_Systems and Collision_Systems have
been moved. The SFML audio system has also not been moved
yet.