map values from sequence to an associated PDB structure's BFactor column

Suggested by Hari Jayam: http://www.jalview.org/pipermail/jalview-discuss/2012-January/000750.html

Basic functionality would be to provide a dialog allowing a specific annotation row associated with a sequence mapped to a PDB file shown in the structure view to be mapped into the B-factor column when the PDB file is exported.