show multiple objects (rebased onto dev_5_0)


This is the same as gh-2972 but rebased onto dev_5_0.

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Fixes a bug with the 'show' logic that did not support multiple objects.

To test:
- Browse to a Dataset and select 2 or more images.
- Use the 'link' button at the top right to copy a link to all images
- Paste the url into the browser url, navigate there and check if all the chosen images are selected. 

@chris-allan I don't know if you'd like this logic somewhere else in the Show class, or if it breaks any of the existing logic. I'll look again at the original PRs for this.