Script authors declare defaults by name strings.
Which can be valid name, or empty string, or "from context".
ScriptFu declares formal arguments to the PDB,
either with a default GimpResource, or defaulting dynamically from context.
Works with both new-style dialogs (ProcedureDialog and ProcedureConfig)
or with old-style dialog (script-fu-interface.c)
Extract methods related to color into new file.
So method names document what is being done.
So related code is together.
Ditto for resource.
No functional change.
Preparing for changes to reset/default and changes to representation of pixels
in lists of differing lengths rgb vs rgba.