Input and Output nodes are changed to Code Block Nodes when Copy/Pasted into Home Workspace: MAGN-4438

@Benglin @ikeough

Inside of the Paste command logic, we detect if the current workspace is the Home Workspace, and if so, we create a new identity CodeBlockNode to replace any instances of Input or Output nodes.
![copy](https://cloud.githubusercontent.com/assets/1699155/4205764/f3d82908-3843-11e4-971e-9412b3fcd97b.PNG)
(Not sure why the `b` output isn't showing in this pic, might be an unrelated bug).

![paste](https://cloud.githubusercontent.com/assets/1699155/4205765/f6047dbc-3843-11e4-92c4-53b608339997.PNG)


```System.Windows.Media.Imaging``` and ```using GraphLayout```? Doesn't look related :)