Prune off file extension when sequences from imported structure files are named after the data file

When a sequences is loaded into Jalview and its accession is unavailable or could not be resolved, Jalview uses the data file name to identify the sequence, this is very rampant for sequences loaded from structure files (especially modelled structure files).

Currently, Jalview includes the extension of the data file in the sequence name. I reckon it would look more tidy if the extension is pruned off.