Drag & Drop of tree leaves from branch to branch

As a user, I need to be able to move leaf nodes from one branch to another branch.

So, for a tree data structure, my initial state:

```

Item 1
  subItem A
  subItem B
  subItem C
Item 2
  subItem X
  subItem Y
  subItem Z
```

I then drag 'subItem Y' from Item 2 to Item 1:

```
Item 1
  subItem A
  subItem B
  subItem Y
  subItem C
Item 2
  subItem X
  subItem Z
```
