If init succeeds, we will have one singleton as before.
If init fails, we will also have one singleton, which will
return an error any time g_initable_init() is called
(thanks to 51b22906).

Previously, if init failed, the weak reference would cause the `static
GObject *self` value inside the constructor function to return to NULL,
meaning the next call to photos_tracker_queue_dup_singleton() would try
to initialise Tracker again. In some cases this could result in a very
slow startup.