Remove script access to deleted references that have no content file

In original MW these objects are permanently deleted and can not be accessed anymore.
Use GMSTs for sound fading distance
Switch sound distance model to AL_INVERSE_DISTANCE
Attach sound listener to the player head instead of camera
Add vanilla-compatible range limiting for playloopsound (Fixes #244, Fixes #1342)
Merge remote-tracking branch 'scrawl/master'