Make WebFlowCredentialsDialog cancellation- and deletion-safe

Make WebFlowCredentialsDialog cancellation- and deletion-safe

- Add new signal to let WebFlowCredentials know and emit asked() to also
  tell AccountState that the user won't authenticate, and triggering
  log-out state in the settings window.

- Use deleteLater() to safely delete WebFlowCredentialsDialog, so
  that Qt can free it at the right time and without crashes.
  Do the same with it's _webView and _flow2AuthWidget on closeEvent().

Signed-off-by: Michael Schuster <michael@schuster.ms>