Add new methods to VerificationRequest (#3441)
* Add new method `VerificationRequest.getQRCodeBytes`
... which requires fewer complicated classes than the existing `qrCodeData`
* Add new property `VerificationRequest.otherDeviceId`
... to save going via `.channel`
* Add more methods to `VerificationRequest`
... to avoid the need for `channel`
* Use new methods in integration tests
