Settings: Hide unwanted elements on the OutgoingPage

As the page is disabled before it is saved we can not use isEnabled() to determine
whether to save the SMTP password or not. isVisible works fine though and the GUI
looks cleaner that way.

REVIEW: 118509
GUI: Prevent a blank line in the Outgoing settings page

It's better to sync the port warning state with whatever is initially in the
settings.

REVIEW: 118523