plug-ins: port file-aa to GimpProcedureConfig

Now takes an integer index into the aa_formats array instead of the
format's name. "4" is about as magic as "weird aallib format name #4"
so no harm done.
5 jobs for master in 21 minutes and 57 seconds (queued for 1 second)
Status Job ID Name Coverage
  Dependencies
passed #459472
deps-git

00:03:50

passed #459473
deps-x86_64-w64-mingw32

00:12:02

 
  Gimp
passed #459474
gimp-autotools

00:09:54

passed #459475
gimp-meson

00:05:44

passed #459476
gimp-x86_64-w64-mingw32-meson

00:09:10