Publishing grey out (rebased onto develop)



This is the same as gh-2458 but rebased onto develop.

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Roll back some modifications introduced while working between version 4 and version 5.
To test:
 * Select 2 images with more than one channels
 * Go to the publishing menu
 * Check that split view is not greyed out.
 * Select the split view item
 * Run the script.
 * Check that the generated .jpg has the 2 images "split"

see https://trac.openmicroscopy.org.uk/ome/ticket/12251