Add default config file menu again

[Skarvion]

Much appreciated for the update to introduce the UX to set the default browser and editor, flows really nice.

That said, I want to set the default screenshot directory, and I find that it's better to uncomment export OMARCHY_SCREENSHOT_DIR in the default config. I do wish it should be possible for us to access the default config file again easily via menu to add new environment variables back here again.

Could argue to add more UI menu for all options, but might be hard to keep up to date with all features