Use local version of plugCubed.js in Firefox extension

The included file has to be in `/data/plugCubed.js` before the extension is built. Only the main script gets loaded from /data, because once the script is running in the page context, it doesn't have access to the plugin resources any more. I also used the unsafeWindow object to check if the API is loaded, which according to MDN isn't a supported API and could change in the future, but they don't recommend an alternative, so I have to keep using it.