PWA / offline mode (including diff data updates)

[rugk]

It would be best if this could…

• be a PWA (that is AFAIK quite simple as adding a manifest.json file or so
• work offline, once downloaded (this requires a PWA AFAIK)
• and now, to get the real gain, cache all downloaded data locally and only do differential updates when needed (i.e. getting all data since the last update)

Problem is of course: That data loading is slow and apparently already a quite big file. So getting a faster loading performance would be the end aim here.

[badlogic]

Happy to merge a PR for this. The data diff may be a bit complex though.