plug-ins: port file-png to GimpProcedureConfig and use propwidgets

Remove the save dialog's builder file and construct the dialog
manually, which is actually less and more obvious code thanks to
propwidgets, which are not suited for a builder GUI.

I guess file-png-save counts as a real test for the new procedure
config stuff, and it seems to work nicely. The only thing that's a bit
ugly is the GimpMetadata integration which doesn't exist yet.
4 jobs for master in 12 minutes and 7 seconds
Status Job ID Name Coverage
  Babl
passed #444366
babl-git

00:02:17

 
  Gegl
passed #444367
gegl-git

00:02:32

 
  Gimp
passed #444368
build-git-autotools

00:07:16

passed #444369
build-git-meson

00:04:39