Separate activity for each desktop in 4.5?

[Josh Stephenson]

In earlier versions of kde 4 (I can't remember when it started it was probably 
4.3)  you could check a box in the activity settings to associate a separate 
activity for each desktop/workspace. This would allow you to have a separate 
wallpaper for each desktop.

In 4.5, with the introduction of the activity manager, this option seems to 
have been moved or vanished entirely.  I  can't find a way get more than one 
wallpaper for all of my workspaces and I can't find a way to merge activities 
and workspaces.  I personally don't have a need for both workspaces and 
activities... and while I understand some people do, taking away the other 
option seems like a regression.

Or maybe I'm just looking in the wrong place.  Does anybody how to achieve 
what I want to do in 4.5?