Currently it's only added to GDBusProxy's g_initable_init() and
g_async_initable_async() methods used by makeProxyWrapper(). But
there are use cases for not using the wrapper, for example to pass
a custom g-flags property; GDBusProxy's convenience constructors
may be more convenient than g_object_new()+g_initable_init(), so
monkey-patch them as well.

#122