Add thumbnail images to offline base layers for visual selector

[mmaelicke]

I approved this PR as it is. However, to be able to cope with future additions of new base layers, we can extend the offline context to sync each base layer image a second time as 64x64 or 128x128 img which can be used as a thumbnail. That's better than static images as we might tend to forget about the images...

Originally posted by @mmaelicke in #30 (comment)

[JesJehle]

Yes, great idea. It was a bit clunky anyway.