Retrieve Uniprot entries for imported PDB structures

Whilst the PDB sequence fetcher option is useful, having retrieved a PDB file, there is then no way to traverse other data sources such as uniprot protein sequences associated with PDB records.

It would be useful to be able to:
1. retrieve and show cross-references (which in this case are 1:1 mappings, ie PDB chain->protein/nucleotide sequence) for PDB chain sequences in a view

2. Retrieve and show Uniprot and Nucleotide sequences for one or more PDB Entries directly from the PDB sequence fetcher.