Autoselect the most appropriate viewer when existing structure views available for selected sequences

When more than one sequence is selected for showing structures in Jalview's structure chooser, if one of the structures is already open elsewhere, the 'Add to view' drop down list of viewers should be pre-configured with the View that contains the selected structure(s).

This is a refinement on what JAL-2944 already offers - since when only one structure view is present, that is already selected in the drop down. For users working with several structure viewers, some intelligence would help avoid mistakes that can't be undone (ie there is no 'remove structure' option for a structure view).