Viewer prev next btns

[will-moore]

This fixes the broken "Prev" and "Next" buttons in the image viewer (were always disabled).
Bug is in 5.2.1, probably as a result of #4242

To test:

• Open a Dataset with multiple images, double click first image to open viewer.
• "Next" button should be enabled, allowing you to move to next image
• "Prev" button should be disabled when viewing first image, but become enabled for other images.
• Check that new Robot test is passing.

[gusferguson]

@will-moore

Tested using https://cowfish.openmicroscopy.org/merge/webclient/ user-3

Behaves as expected.
Don't know how to check robot tests.
Otherwise good to merge.

[mtbc]

Robot tests: https://ci.openmicroscopy.org/job/OMERO-DEV-merge-robotframework/ is green.