wireless-security: remember username/password across pages

Several different WPA Enterprise authentication types use username and
password, but each type has its own set of widgets in the editor, so
if you switch types, you lose any username/password you typed in
before. Hack this by having the WirelessSecurity object record the
last-seen username/password, and having the EAP widgets read from
that when they're shown, and write to it when they're hidden.