plug-ins: port file-png-save to begin_export()/end_export()

which removes quite some boilerplate. Also remove the boolean
"profile_saved" return value of save_image() and instead modify the
config. Add a "bits_per_sample" return value and set value on the
metadata instead of always "8".
5 jobs for master in 28 minutes and 2 seconds
Status Job ID Name Coverage
  Dependencies
passed #461460
deps-git

00:04:03

passed #461461
deps-x86_64-w64-mingw32

00:15:10

 
  Gimp
passed #461462
gimp-autotools

00:12:51

passed #461463
gimp-meson

00:06:36

passed #461464
gimp-x86_64-w64-mingw32-meson

00:07:10