Pjsua2 Swift sample project bug fixes & feature updates.

[emretufekci]

• NAT Settings
• Saving configurations and fetching it from local db.
• Scroll Bar

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[sauwming]

Thanks for the enhancements, but it's quite complicated to be integrated as a simple sample app. Nevertheless, we'll refer our users to this project if they need an example of a more full-featured pjsua2 swift app.