Abiltly to have multiple Image Viewers open

[IanMayo]

The ImageViewer plugin is configured to allow multiple instances.

But, the Eclipse UI doesn't offer the ability to create new instances - it has to be done programmatically. (it's worth checking this, but this is what I found).

We should add a New Image Viewer command button to the Image Viewer context menu. This will create a new instance, with a new ID. The IDs have to be unique - so I guess we could use date/time.