Scriptfu improves

Addresses #5426 (closed) and #6255 (closed)

Note that #6255 (closed) has an attached patch. This merge request duplicates the patch. So don't apply both this MR and #6255 (closed). This MR can close #6255 (closed).

Note that #5426 (closed) also had an attached patch. It was wrong and should NOT be applied. This MR can close #5426 (closed) (unless I missed some obsoleted PDB procedure, or more become obsoleted.)

My plans: I also plan to fix #6026 (closed) and #5402 (closed) (which also has a patch attached that I can redo in the form of a MR.) But I would prefer the org to apply this MR, so that I can see that you trust my ability to work on scriptfu (you might prefer that Kevin works on it?) and so that I don't have to keep track of so many branches, and so that anyone doing testing (2.99 users) won't see or report error messages that can already be fixed.

Future changes do not depend on this MR, but this MR (especially the enhances to the debug/error messages, which are quite extensive) makes it easier to read and change the code.

To test the #6255 (closed) changes: export G_MESSAGES_DEBUG=scriptfu and then run GIMP, something that calls scriptfu. You should see g_logging style messages from scriptfu in the console.

To test the #5426 (closed) changes: run GIMP and call one of the plugins listed (script_fu_distress_selection). It should succeed (it did not before) and you should see 'scriptfu-WARNING ....is deprecated, please use ..."