Empty "Color" and "Highlighting" fields after changing of an alignment alphabet

When alphabet of a sequence, added to a multiple alignment, differs from alphabet of the alignment, the alignment alphabet may be modified. This use case is not currently taken into account by "Color" and "Highlighting" items on the options panel.

For example, to reproduce the issue:

1. Open "data/samples/CLUSTALW/COI.aln".
2. Add an addition sequence from a file, e.g. amino acid sequence "_common_data/fasta/amino_ext.fa".
3. Open "Highlighting" option panel tab.
   Actual result: items in the "Color" and "Highlighting" combo boxes correspond to the obsolete alphabet.
   Expected result:
   • Alphabet of the alignment was updated (to "Standard amino acid" alphabet in this case).
   • Items in the "Color" and "Highlighting" combo boxes correspond to the values of the new alphabet.
   • The values of the combo boxes are set to default for the current alphabet.
4. Close the Alignment Editor.
5. Open the view again.
6. Open the "Highlighting" tab.
   Actual result: "Color" and "Highlighting" combo boxes are empty.
   Expected result: Values of the combo boxes are the same as on step 3.

Fix the issue for all possible modifications of an alignment alphabet (Standard DNA > Standard amino acid, Standard RNA > Raw, etc.).