Add internal GimpProcedureConfig API to load/save "default values"
which are to be treated as if they were the hardcoded GParamSpec
defaults, but user-configurable. Also make all other load/save
functions available to other libgimp files.
In gimp_procedure_run(), if incomplete arguments are passed, don't
just complete them with the GParamSpec defaults, but look up the
user-saved defaults and use them if they exist. This happens before
everything else and brings back the PNG export feature of using
user-saved defaults also in non-interactive mode (but for all
procedures not just PNG export).
In GimpProcedureDialog, add "Load Defaults" and "Save Defaults"
buttons, they are the only way of managing the user-configurable
procedure defaults.
When clicking "Reset", show a popover with the reset options "Initial
Values" and "Factory Defaults".
