Warning dialog before deletion of linked images

If the user wants to delete images which are linked to multiple datasets a warning dialog is shown.
Ticket: https://trac.openmicroscopy.org.uk/ome/ticket/11883

To test:
Copy an image from one dataset, paste it into another dataset, then try to delete the image.
This following dialog should be shown:
![image](https://f.cloud.github.com/assets/6575139/2396234/2fc1d9ca-a9d6-11e3-85a0-18bff50b52c1.png)

Putting this up for discussion. Required quite a lot of changes, if there would have been an easier way to implement this, please let me know.