The `VerificationRequest` object is used to control the flow of the
verification but only up to a certain point.
Once we start handling of different specific verification flows (i.e.
SAS or QR code verification) the `VerificationRequest` object creates a
child object of the Verification type.
This patch adds a new `VerificationRequestState` variant called
`Transitioned` which holds the child verification object as associated
data.
This makes it much simpler to go through the whole verification flow by
allowing users to just listen to the `VerificationRequest::changes()`
method.
Damir Jelić
e9c3aa1a2e