This allows our script-fu plugins to use the GIMP enum values, just like all our other plug-ins know them.
In other words:
- Names are consistent with those of other plug-ins
- Introspectable enums are automatically added as they are added to the introspection file, without even needing a recompile
- No need to keep track of our enum types anymore, as that is just a redundant implementation of introspection in practice. This in turn will let us simplify some of the pdb code