Add save-username checkbox in the connect dialog.

* changes
  + Add checkbox to layout file and handling code in related classes.
  + Add new methods to VpnProfileActor.
  + Move dialog handling to VpnSettings from AuthenticatorActor in order
    to support screen orientation change.
  + Hide "Connect" in the context menu if the profile is connecting.
  + Enable connecting profile in case it was disabled in last call.