Saveload: Add SaveLoadStructHandler handler for post table header save

src/sl/saveload.cpp

@@ -2440,6 +2440,7 @@ SaveLoadTableData SlTableHeader(const NamedSaveLoadTable &slt, TableHeaderSpecia
 					std::unique_ptr<SaveLoadStructHandler> handler = sld.struct_handler_factory();
 					sld.struct_handler = handler.get();
 					sld.struct_handler->table_data = SlTableHeader(sld.struct_handler->GetDescription());
+					sld.struct_handler->SavedTableDescription();
 					saveloads.struct_handlers.push_back(std::move(handler));
 				}
 			}

src/sl/saveload_types.h

@@ -273,6 +273,11 @@ public:
 	 * Called immediately after table_data is populated during header load.
 	 */
 	virtual void LoadedTableDescription() {};
+
+	/**
+	 * Called immediately after table_data is populated during header save.
+	 */
+	virtual void SavedTableDescription() {};
 };