Add better autosave functionality

[ainslec]

(betabeta report)

Getting reports that users are losing work randomly when they close their browser window without saving first. Potentially hours of work can be lost.

Think this is down to indexeddb not immediately saving on exit hook.

options:

1. Force Adventuron to save game source code to disk every 20 minutes (if there has been an edit).
2. Fix the indexeddb code (repplicate the error first to test this).
3. Make Adventuron based on the new file API, where a user has to select a filename the first time they save a new gamefile.