gobject: Serialize properties holding a GObject
gobject: Serialize properties holding a GObject

Like we deserialize them, we can serialize GObject properties
defined using GParamSpecObject.

GNOME/json-glib,ebassi/json-glib,brauliobo/json-glib,robtaylor/json-glib-gvariant,oerdnj/json-glib,oerdnj/json-glib,Distrotech/json-glib,brauliobo/json-glib,frida/json-glib,oerdnj/json-glib,GNOME/json-glib,Distrotech/json-glib,Distrotech/json-glib,oerdnj/json-glib,frida/json-glib,ebassi/json-glib,robtaylor/json-glib-gvariant,ebassi/json-glib,brauliobo/json-glib