Make available colour schemes dependent on sequence collection

Currently hard-coded:
- nucleotide colour schemes are enabled in the Colour menu if alignment is nucleotide, else disabled (except for 'Nucleotide'!!)
- this logic is missing in the pop-up menu for a selection group
- peptide colour schemes are always enabled (although not applicable to nucleotide data)

Three possible improvements:
1) enable/disable nucleotide colour schemes also in the pop-up menu
2) enable/disable peptide colour schemes in AlignFrame and pop-up menus
3) drive this by a property of the SequenceCollectionI rather than hard-code lists of colour schemes

1) and 2) are straightforward. Adding methods isNucleotide(), isPeptide() to ColourSchemeI would help.
3) needs some thought:
- SequenceGroup doesn't have a method isNucleotide(), preventing adding this to SequenceCollectionI
- could perhaps abstract a bit further e.g.
    ColourSchemeI.isApplicableTo(SequenceCollectionI)
    - with current implementation limited to the nucleotide / peptide check