annotation colouring should allow switch between 'use alignment colours' and 'use colour from annotation'

Currently, Jalview's UI interprets the setting of the Annotation Color dialog's 'Use original colours' checkbox to 1) use the annotation's values + defined threshold to filter of an existing alignment colourscheme, and 2) shade sequences by colours from annotation row - e.g. red/green helix/sheet colours or PLDDT shading (JAL-3895).

This is problematic when trying to shade alignments with a pre-existing colourscheme to one using annotation colours, since it is first necessary to set the alignment colouring to 'None'.