Add support for Pre-Shared-Key (PSK-TLS) connections

[tringenbach]

• Option added to Certificates dialog even though it's not really a cert
  • Seemed like the best / most "TLS advanced option" place After all, it's used INSTEAD of a cert
• Uses the username as the identity
  • a future advancement might add a pskIdentity field, since technically it doesn't have to be the same
• Requires unchecking Verify TLS
  • There's no cert to verify when using PSK