Implement UI for device verification

[MidhunSureshR]

• [ ] Extend the toast collection code to support arbitrary types of toast messages
• [ ] Have a button to start the emoji verification under cross-signing in the settings view
• [ ] Do the verification in the middle panel
• [ ] Implement some sort of event emitter on the SASVerification class so that the UI can subscribe and react to it
• [ ] Sign your device when SASVerification completes