Remove getResidueBoxColour from api.SequenceRenderer

Calls to SequenceRenderer.getResidueBoxColour are always followed by a call to findFeatureColour (unless feature display is turned off).

This logic is already encapsulated in method SequenceRenderer.getResidueColour() so it would be simpler always to use this method instead.

Remove getResidueBoxColour from the api and change all calls to it to just call getResidueColour. This method now accepts a FeatureColourFinder (JAL-2438) so it can do the feature colour lookup after the colour scheme lookup.