Paging: Refactor producers into classes?

[dbrgn]

I had some concerns regarding the original implementation (009d6eb), especially because the client (browser) could control so much information (template path, list and stat producers that didn't even have to match). Seems better now but I still wonder if it wouldn't be better to define a base class with two methods that a paging producer implements instead of registering two functions each.

See 2ad9547#commitcomment-6448151