Fixed highlighting bug after cancelling the addition of something to a list slot.

The empty slot in a list slot used to remain highlighted if you hit cancel on the constructor arguments panel after dragging to fill. I fixed it by moving the call to clearhighlightdroppables so that it doesn't get lost by falling into some hole in that block of if statements. 

Note that now, the slot pane highlighting behavior matches the behavior of the dataflow pane -- namely, the un-highlight occurs before the dialog box appears.