Port to 5.x-2.x

Heya - after a long time, the 5.x-2.x branch is now back to fully working. While there might be the one or the other minor change still (I consider a theme function for revisions that all backends need to provide), the 5.x-2.x API is pretty much finished and the backend should be ported as soon as possible.

Please have a short look at the alpha1 release announcement and a longer look at the API changes documentation page - there's lots of stuff that should make the backend much cleaner. I'm pretty busy with other stuff, so I can't really help out a lot (unfortunately) with the 5.x-2.x port for the backend. I do hope it'll get done still - with a little luck, the 6.x port will be a breeze afterwards :)