Fixed dragging conversation tabs

Yay, an easy one! Assuming I got it right.

I realized that when dragging a tab, an error surrounding the minDragDistance was occuring. You'd click-and-hold a tab near the edge of another tab, then "drag" into that nearby tab. However, the drag only actually started after the minDragDistance, causing you to drag the nearby tab instead of the one you clicked on.

So this change is keeping track of the tab you clicked on.