Support File API in Web variant #154
Description
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 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).