Core seams placeable by editor (or remote interface)

Right now core seams are generated by script and cannot be placed in the editor. Some players want to place the seams using the editor (for various reasons - migrating a save or testing or "cheating"). This currently doesn't work so we should make the core seam code respond to the placing of core seams (in an on_entity_created/cloned handler) and update the SE tables with the relevant information to make them functional.