allow free text search/association to add 3d structure data to sequences

Currently, if Jalview cannot resolve a sequence against Uniprot or the derived sequence ID text yields no useful matches in PDBe, there is no option shown in the 3D structure chooser dialog except load file and manual input of PDB/chain IDs. It would be great to have the option to perform free text searches (like the PDB-FTS dialog currently does) but instead of adding new sequences, the structure data is associated with the existing (selected) sequences. This could be done using Jalview's regular sequence<>structure mapping mechanism, where the structures that conservatively match sequences are linked.