This moves the details of dialogs that may be needed when accessing secret
storage to centralised helper. In addition, this clears the secret storage key
cache so that keys are only live for a single operation.
This adds dialogs for creating and accessing secret storage via a passphrase or
recovery key. These flows are adapted from the ones used for key backup.
This is not part of any designs, so it may be short-lived, but it's quite handy
for diagnosing issues with cross-signing at least while the feature is in
development.
