random helix colour palette for colour by annotation with RNA secondary structure

The colour by RNA Helices function available since jalview 2.7 generated a random palette of colours with which to shade columns annotated with distinct RNA helices. In Jalview 2.8.2, per-sequence RNA helix colouring was introduced, but unlike the RNA Helices shading model, the helix colour palette was randomly generated only once per Jalview session.

This minor improvement allows the RNA Helices shading scheme to be randomly picked every time the colour-by-annotation dialog applies an RNA shading style to the alignment.