'Disordered region' secondary structure state

Disordered residues in 3D structures are currently visualised in Jalview's structure derived secondary structure annotation line as an *absence* of annotation, and thus indistinguishable from regions of the associated sequence excluded from the structure determination.

'Not observed' regions are marked in SIFTs records, but those are only visible as sequence features - not annotation rows. One option would be to introduce a new glyph and 'Disordered' symbol to highlight those positions and allow them to be used for annotation-based selection.