Support File API in Web variant

The Web variant of CSIDE is currently entirely dependent on the Dropbox API; users need a Dropbox account and files are stored in their Dropbox. This has worked well historically, but it's something of a barrier to entry and makes the implementation of other features problematic (e.g. #153).

It would be great to fully support the [Web File API ](https://developer.mozilla.org/en-US/docs/Web/API/File_API) so that users can use CSIDE Web to edit local files, compile their games and perhaps even run automated tests from their web browser (not to mention avoid logging in).