Support export/import of semantic domain questions

[imnasnainaec]

#3081 introduces custom semantic domains (with id, name, language, description, and questions). LIFT import/export between FLEx and The Combine handles all those details expect questions. We would like to (at the very least) import/export questions from The Combine in a way that fits the LIFT schema and doesn't interfere with FLEx imports. Additionally, we could modify the general LIFT library code to optionally support questions (default: not included).