Prevent the app from displaying multiple identical popups

[fbnlsr]

Is your feature request related to a problem? Please describe.

When the "Notify on finished conversion / Display message" is set to true and the Verifiers / AccurateRip Verifier Notification is set to true as well, there can be as much as 4 popups appearing on screen, with three of them displaying the same "All tracks successfully verified with AccurateRip" text:

• One when the "ripping/decoding" step is done
• One when the "encoding" step is done
• One when the "verifying" step is done

Finally, the fourth one with the default text "Conversion process finished!"

Describe the solution you'd like

It'd be a much better UX to have one single popup with a summary of what's happened.

"Conversion process finished! All tracks successfully verified with AccurateRip."