Clustalx colour scheme should be renamed (back) to Clustal in the GUI

The MSA window "Colour" menu lists "Clustalx" as a colour scheme.
When running Jalview headlessly and applying a colour scheme on the command line (e.g. `-colour clustal`) the correct scheme name is "clustal" and not "clustalx" (which produces a purple colour scheme).
For technical reasons "Clustal" is more appropriate for the colour scheme, and would be consistent with the CLI name.

Reported by [~j.abbott]