Object resolve should consider prototypes of GObject interfaces

It's currently not possible to add or monkeypatch methods on a GObject interface:

const {Gio} = imports.gi;
Gio.File.prototype.test = 42;
const file = Gio.File.new_for_path('.');
print(file.test);

This is undefined and not 42. (new_for_path() actually returns you an instance of GLocalFile which is a class that implements the Gio.File interface.)

The resolve hook in object.cpp should, when searching the prototype of a class, also search the prototypes of any implemented interfaces.

/cc @llzes