Although I wasn't able to find a collapse-all solution, I did find a work-around that might help you out.
You can move all of your layers into a single major layer, and then alt-click the expand/collapse arrow next to the newly-created major layer. This should collapse (or expand) all of the layers and sublayers within that major layer.
I'll keep poking around to see if I can find another solution.
Europe, Middle East and Africa