Draft: Enhancement #8493 parse mock enums from g_param_spec blurb

In this MR, see changes in devel-docs/porting-guide/script-fu-author-guide.md which describe changes from the author point of view. For a Python plugin, it would be similar, except the author would code the G_PARAM_SPEC_INT with a blurb as described, and max/min.

The MR is a draft until the concept is reviewed. Other alternatives have been proposed. @Jehan please review, decide whether the approach is adequate. Don't review for style.

If the concept is approved, I will finish the MR. The MR works but needs more testing and more coding on the ScriptFu side. Filter>Blur>Tile in the repo is a test case.