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.
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