Allow injection of custom structure viewer script when viewing 3D data from Jalview

Provide a way to specify via command line or via the GUI a fragment of structure viewer specific script that Jalview will send to the viewer.

Jalview already has a 'Viewer colouring' mode, where it doesn't try to modify the viewer display in any way, but other types of customisation would be useful.. eg. provide the script fragment executed everytime a mouse over occurs or an old one is removed, or the script that is executed to control the rendering style (cartoons or otherwise currently) for imported or mapped structure data.