"Quit" option when working directory is missing

[Haeleon]

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.

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