2.99 plug-ins: fix to use new GimpResource

This is part 3 of the larger "make GimpResource class"

Part 1 is !740 (merged), add the class, leaving GUI and plug-ins broken.

Part 2 is !771 (merged) fix the GUI around chooser widgets for resources

I intended the parts to be cleanly separated, but the latter parts sometimes fix bugs in the earlier parts. Each part branches off the preceding part. Each of the three parts are one squashed commit in separate branches. I rebased the branches (Part 2 rebased to Part1, then Part3 to Part2.) So I think that Part 3 alone needs review and merging. I think you can review the individual parts, but again, there might be a few fixes already in following parts.

(Somehow I got two commits for Part 2 ???)

@Jehan Ready for review. Thoroughly tested, but I will test again after I get it to pass CI. I think it is very clean now, but of course it is so massive that there are bound to be problems.

Also note that the app/core side remains to be simplified: the chooser widgets are showing more attributes and passing them to libgimp, and libgimp simply ignores all but the chosen resource ID.

I plan to work on ScriptFu, then possibly the item chooser widget, as discussed elsewhere.