Rather than looping through all threads, use the references property
to find the parent directly. It should be guaranteed to exist since
one cannot reply to something which does not have an id.

Furthermore, when there is no parent reference, the root serves as
the parent. Without considering this, deleting top level annotations
leave stubs in the UI.

Add some tests that the events do what they're supposed to. NB: the
thread method itself should probably be tested.