Remove settings when uninstalling

[Cuperino]

Is your feature request related to a problem or a limitation? Please describe...
Uninstalling doesn't remove app settings, making it less effective as a troubleshooting technique.

Describe the solution you'd prefer

• On Windows, provide a checkbox for users to remove/purge their settings while uninstalling.
• On Linux, ensure purge removal successfully deletes QPrompt's settings.
• On MacOS, ??? Investigate what are the conventions there.

Describe alternatives you've considered
Factory reset option inside the app. This option wouldn't be of any help if the user is having trouble starting QPrompt because of rouge settings.

Provide use examples
An invalid setting prevents QPrompt from starting. User can uninstall and reinstall with its settings cleared to fix the problem.