Allow creating new variables directly within a variable sub-group

Currently, newly created variables always appear at the root of a variable group. When you have sub-groups within variable groups this behavior gets annoying fast, because it means scrolling up from the sub-group to the top of its parent, finding the newly created variable, and manually dragging it down into the appropriate subgroup. If the sub-group list is long and a lot of variables must be created, this can be a tedious exercise.

It would be fantastic if we were able create a new variable at any level to eliminate the need for this manual step.