plug-ins: port file-fli to GimpProcedureConfig

While testing noticed that the plug-in either loads or saves total
garbage, but also in 2.10, so it's not a recent regression.
5 jobs for master in 24 minutes and 29 seconds (queued for 1 second)
Status Job ID Name Coverage
  Dependencies
passed #455014
deps-git

00:02:59

passed #455015
deps-x86_64-w64-mingw32

00:14:16

 
  Gimp
passed #455016
gimp-autotools

00:08:35

passed #455017
gimp-meson

00:05:03

passed #455018
gimp-x86_64-w64-mingw32-meson

00:10:12