"Quit" option when working directory is missing

### Enhancement Summary

I use cloud storage for my main folder, and before it's loaded, the folder doesn't exist. If I launch LMMS before it's ready, a popup asks if I want to create the directory. Hitting OK tries to create a new folder, and hitting Cancel or closing the popup changes the setting. It would be nice to have a "Quit" option that simply stops the launch without changing anything.

### Implementation Details / Mockup

On the popup asking to create LMMS's working directory, add a "Quit" button that closes the program with no additional effects.

### Please search the issue tracker for existing feature requests before submitting your own.

- [x] I have searched all existing issues and confirmed that this is not a duplicate.

Find the startup popup that appears when LMMS's working directory is missing and inspect the existing create and cancel handling. Run LMMS with an unavailable working directory to verify the new Quit option exits without creating a directory or changing the setting.

Written by the indexing model from the issue text.