Implement file select dialog

[Sintrastes]

In order to implement importing of JSON files, we'll need to implement some sort of "file import tool".

This should be fairly simple to do with JS, using the built-in browser capability to import file (though I'm not sure how easy this will be to do on Android.

See: https://stackoverflow.com/questions/16215771/how-to-open-select-file-dialog-via-js