Single ch split 2923 lineplot 2924

A couple of minor tickets addressed:

https://trac.openmicroscopy.org.uk/ome/ticket/2923 Open a single-channel image in the main web image viewer. Check that the "Split Channel" option is disabled. Check it is not disabled for multi-channel images.

https://trac.openmicroscopy.org.uk/ome/ticket/2924 Don't allow the Line Plot to be shown in split-view or max-intensity projection. This was already fixed in the code (no changes) but just check it's working now that I'm closing the ticket: Choose "Split Channel" or "Max Intensity" view and check that the Line Plot is disabled.