app: widgets: fix #11422 prop widget not support GeglParamColor

Alxsa: I leave it to you to review and merge. Or you may want to involve Jehan.

Also, the changes I made are not how you described the needed changes in the issue comments. I.E. it is not doing any "converting ... then back." But maybe your description was just high-level, off the top of your head.

Very lightly tested.

Re utility routine, other developers might just duplicate the code in app and libgimp. I think this is sufficient for now, since otherwise, many filters remain broken. Later, we can revisit how we share versus duplicate code re param specs between app and libgimp.

I tackled this because the GeglColor changes require more changes in ScriptFu, and doing this helped me clarify what those changes might be.