Add a command-line option for resetting application settings

[sergm]

It should be possible for the user to revert the application settings to the factory defaults, even in case the application no longer starts normally, so it may become challenging for the user to adjust settings any longer.

This is sometimes done by adding a "purge" function to the uninstaller. However, it is not a convenient option if the user wants to continue working with the application. Additionally, if the application is installed system-wide, the uninstaller typically fails to purge settings for all the users who have them created, and performs the cleanup for the current user only.

Hence, it should likely be a command-line switch that the application parses and handles early, e.g. before attempting to start MIDI and audio processing, which might block normal startup in some rare cases (albeit, such bugs deserve a separate issue).