Add support in GimpProcedureDialog for plugin defined mock enums
Re SF-OPTION in ScriptFu and PF-OPTION in v2 PyGimp.
These were used to declare arguments of a plugin procedure that are mock enums. They should display in the a plugin's dialog with an int combo box having a small int store. The plugins receive the underlying integers.
Currently, in GIMP 3 these don't work for plugins that use GimpProcedureDialog.
This has been implemented in some C plugins implementing their own GUI, by stuffing the enum description into the blurb of a g_param_spec_enum, and creating an int store from the blurb, to pass to an int combo box.
The enhancement is to make GimpProcedureDialog do the same thing (parsing the blurb.) Then ScriptFu and other language plugins using GimpProcedureDialog will show proper widgets for the args declared as mock enums. The PDB Browser already displays the blurbs, showing enough that a script developer can understand.
A plugin's code uses integer literals, not symbolic enum values. This doesn't actually define a real enum type.
This is a step towards Python and ScriptFu plugins using GimpProcedureDialog instead of building their own GUI.
See also !709 a proposal for an alternative implementation which would create real enums and might let script authors refer to enum values symbolically.