> IMessageBox was changed up a bit in #4006
While that was not the general intention of the PR, some cruft remains, namely the `bool?` parameter and return value on `Confirm` as well as the missing `caption` argument on `Message`.
The `bool?` should be replaced with a thin wrapper over the `DialogResult` members `Yes`, `No` and `Cancel`.
`Message` should get a (possibly optional?) parameter `caption`. If necessary, callsites need to be updated.