Combine images param unwrap 11473

Fixes https://trac.openmicroscopy.org.uk/ome/ticket/11473.

To test, choose 2 (or more) single-channel, single time-point images, Run util script > Combine Images.
De-select the "Auto Define Dimensions", select "Manually Define Dimensions" and choose Dimension 1: Channels.

Optionally choose colours and or names for the channels.
Run the script.
Check that all the input images got combined into a new image.