object: Support external construction of gjs-defined GObjects

Currently GObject subclasses defined in JS must be instantiated from JS
constructors, as that's where the association of the newly created native
object with the JS context and wrapper object happens. Allow this code to
run as well for GObjects that are instantiated externally via g_object_new()
(e.g. by GtkBuilder, gom, json-glib, ...) by creating the wrapper object
from the ->constructor() vfunc in that case.

https://bugzilla.gnome.org/show_bug.cgi?id=681254