Colour sequence by Aggregate / Consensus of all sequence associated annotations

JAL-2226 introduced the creation of distinct annotation rows for each chain in a multimeric/multicopy structure mapped to a sequence.

This breaks the existing colour-by-annotation system, because it means one cannot simply open the Colour by annotation menu, then after selecting 'Per Sequence' choose 'Secondary Structure' from the dropdown menu.

Ideally, colouring would take the most appropriate value from each of the annotation rows with the given name per sequence. At the very least, the first non-null value.

Optionally this could yield some form of consensus annotation row for the set.