Add ability to export and import configuration

[j6s]

It would be nice to add an option that allows the user to import and export his entire configuration to a single file. At the moment I have to gather all of my layers correctly, paste them into the correct textarea on the website and import my fn-layer correctly to make a small change.

The ability to import all of that at once would enable me as a user to make small changes more quickly without having to configure everything everytime.

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Slightly related: Integrating a 'change' link to keyboard-layout-editor.com and saving the users last known configuration in localStorage would make the whole process even smoother.