Draft: Restore capability of ScriptFu scripts to call Gegl filters

Draft, proof of concept. Review only for the architecture and concept. Names, files, style not appropriate, not ready for review.

Related to #12279 (closed)

Restores the capability for SF scripts to call Gegl filters, lost when compatibility PDB procedures were deleted for 3.0rc1.

To test, in newly started gimp, open an image (so you have drawable ID 2.)

In SF Console, enter

(gegl 2 "gegl:ripple")

where 2 is a drawable ID. Expect the open image to be rippled. Note other args are mocked up, but are like:

(gegl 2 "gegl:ripple" "amplitude" 10.0)

Where the script author provides name/value pairs.

Currently there is little semantic checking on the SF side, but we could by calling gegl_operationo_list_properties and cross-checking.