GSettingsBackend - Fix thread-safety during destruction of GSettings instances while notifications are emitted

g_settings_backend_watch() uses a weak notify for keeping track of
the target. There's an explanation why this is supposed to be safe but
that explanation is wrong.

The following could happen before:

1. We have the target stored in the watch list
2. The last reference to the target is dropped in thread A and we end up
   in g_settings_backend_watch_weak_notify() right before the mutex
3. g_settings_backend_dispatch_signal() is called from another thread B
   and gets the mutex before 2.
4. g_weak_ref_init() is called on the target from thread B, which at
   this point has a reference count of exactly one (see g_object_unref()
   where it calls the weak notifies)
5. Thread A continues at 3. and drops the last reference and destroys
   the object. Now the GWeakRef from 4. points to a destroyed object. Note
   that GWeakRefs would be cleared before the weak notifies are called
6. At some later poi...