#7 Can only drag a variable when it is selected

[Peter Crowther]

Minor UI niggle: a variable has to be selected in the
list of variables before it can be dragged.

To reproduce:
- Create a new question
- Create variable P
- Create variable Q. Note that Q is highlighted in
the list.
- Try to drag P onto the question area. You can't.
- Click P to highlight it.
- Try to drag P onto the question area. You can.