Add rudimentary support for detached agrp files (audiogroup1.dat etc)

This was made in quite a hurry, code's currently a mess and should be
cleaned up soon(tm).

Basically, it's another FORM, with a single chunk, AUDO, containing a
count-offset list of {uint length; byte WAV_data[length];} pairs.

Actually, it *might* work using the GMFile exporter, but I don't know
(but maybe I should've tested this before writing this crap).
2 jobs from master in 1 minute and 57 seconds
Status Job ID Name Coverage
  Test
passed #115039906
debug

00:00:59

 
  Deploy
passed #115039908
release

00:00:58