Promote PDB ID cross-references to primary references when retrieving 3D structure data

If sequences are named according to PDB IDs, the 3D structure chooser provides perfect matches to the PDB, but in 2.10.1, hitting the 'View' button causes Jalview to perform a 'Fetch standard DB References' for the sequences, before finally downloading the PDB IDs. This can take some time, and seems pointless since the appropriate IDs are already shown in the structure chooser.

Before trying the 'Fetch standard DB References' process, if PDB accessions were discovered based on sequence IDs, Jalview should try to match those first.