Leave space around codon consensus labels under histogram to improve readability

The codon consensus displayed on a protein sequence alignment in a cDNA/Protein view is shown as a histogram where labels are three letters wide, rather than the normal 1, and so the label is shown scaled to fit within the available column, if it is wider than the available space.

For histogram labels, this isn't ideal. Jalview scales them precisely to fit within the bounds of the character width, but actually needs to make then slightly narrower so each label appears distinct from adjacent columns. Otherwise, the labels appear to run together, since consecutive characters are spaced wider than characters in adjacent column labels.