Allow user to specify 'original colourscheme' or colours provided by annotation row independently

Update for 2.12:
This requires a rearrangement of the 'use original colours' option logic such that it allows conservation style shading, or even employ a complex graduated scheme, rather than simply re-using the existing shading but *only* when there is no annotation row specific shading provided. The annotation shading mechanism has been recruited for additional purposes (Hmmer posterior bleaching JAL-3623, PAE groups JAL-4158) and will become more sophisticated in the future, so a UX review is now a priority.

Original story:
Currently, the AnnotationColourScheme logic supports three modes of per-Sequence or single row based alignment/group shading:

i. min/max linear gradient
ii. Reuse of existing 'Background Colour'
iii. Use of colours defined in alignment annotation row.

The Annotation Colouring dialog allows the user to switch between i and ii but iii is currently only available for alignment annotation programmatically created (e.g. via a T-COFFEE file import).

The ability to switch between all three modes, with out data loss, could be useful, but use cases are needed (e.g. toggling between a linear scheme vs the predefined colourscheme provided by T-COFFEE, etc).