Allow multiple PDB structures to be shown without performing a superposition

Jalview currently performs a 3D superposition when it adds a structure to an existing structure view, or when it loads several structures at once. This isn't always useful - particularly if the structures were already superimposed in a special way. There should be an alternative option provided - either as an alternate 'OK' button or a checkbox which allows the superposition command to be suppressed when opening multiple/or an additional structure.