import sequence data from existing structure viewer session

Often, people will already have an existing session in their favourite structure viewer (Chimera, ChimeraX, Pymol, Jmol, etc...), but want to use Jalview to perform more detailed sequence analysis.

For Chimera, for instance, this is 'sort of possible' by exporting sequence alignments from a running chimera session, then importing them to Jalview, then finally associating the sequences with pdb data and opening a new chimera session from Jalview, but that takes a long time, and often doesn't actually work due to problems with sequence/structure mappings.

Instead, it should be possible to open a new linked structure viewer instance from Jalview, import a session into the new instance, and provide options for importing sequences directly from the loaded structure data as alignment views in Jalview.