Just to make sure you are aware, Expand/Collapse all is already available on the List tab of the ribbon. It isn't clear to me from your initial post whether you had seen that there is some level of this functionality already in place. (At least, there is in 6.2.2.)
However, you can only do one or the other (specifically, I think you can do whichever you didn't do last, starting with being able to expand), so you always have to expand once before collapsing; and the List tab doesn't stay selected when you click in the list, so you keep having to go back to it. This feature request sounds like an outstanding idea to me.