Draft: 2.99: plugins define dynamic GEnums, and GIMP keeps them in the PDB

This is a very crude prototype.

See a discussion on the gimp-dev list re SF-OPTION.

The problem it is solving is that plugin dialogs implemented by GimpProcedureDialog do not support arguments formerly declared using PF_OPTION and SF-OPTION. In other words, enums declared by plugins, that are dynamic, that GIMP doesn't understand.

@Jehan could you give this and the gimp-dev discussion, a brief glance? Let me know if I should abandon it now. I might instead work on the alternative solution that you have proposed, if you send me a sketch of that solution. Your idea might be simpler and smaller.