Drag-and-drop items within the StagingView #393

In #391, we floated the idea of supporting drag-and-drop among the lists in the StagingView. I'm forking it off to its own issue here to keep it from getting lost in the shuffle of the more immediate work 😄

Some comments from @kuychaco:

👍 on the drag-and-drop feature though we'd have to put in some extra thought to make it work smoothly with the current feature to drag-and-select-multiple-items. And drag-and-dropping would only work to/from certain lists. For example you can't drag-and-drop into the Merge Conflicts list, and from there you can only drag-and-drop into the Staged Changes list.