Declared defaults for Resource arguments are not effective

This covers related issues #10617 (closed) #10822 (closed) #10856.

Re object-valued arguments Resource (Brush, Font, etc.) Primitive valued arguments do reset properly. Color valued arguments might have the same issue but hard to know since Test>Sphere3 is currently broken for Color widgets.

Symptom: the dialog for a plugin doesn't initially show the default that is declared by name by the plugin for e.g. Font "Sans-serif", after a clean install.

Also, the "Reset>Factory defaults" button of a plugin dialog doesn't have the intended effect. Usually the erroneous default is from the Context, not as declared by the plugin. For GimpProcedureDialog based GUI but also for old-style ScriptFu dialogs.

Only ScriptFu currently lets you declare a default Resource by name. It worked in v2 but doesn't work in v3. Other language plugins don't have a way to declare a default by name (e.g. in PROC_ARG_RESOURCE macro. )

We might also let a plugin declare a default for "other/second" Image by name. But for now, there is no ImageChooser widget in GimpProcedureDialog.