Create features across visible portions of a sequence

Currently, creating a feature from a selection results in one feature created for each sequence in the selection, which spans the beginning and end of the selected region.

If hidden columns are present within the region, and they contain residues, then these residues will also get annotated with the feature. It should instead be possible to by default exclude any hidden sequence data from the feature, either as a user preference and via the Create Sequence Feature dialog.