gegl, operations: refactor gegl-op.h's enum_value() macro

Change it to take 3 parameters:

  enum_value (value, nick, name)

- "nick" is an untranslated string for (de)serializing the enum value
  that must contain no spaces.
- "name" is the human-readable name for the GUI, wrapped in N_().

Change enum registration to translate the name if GETTEXT_PACKAGE is
defined.