Default engine always used for loaded queries

When a query is loaded from an existing project, it is always assigned the default engine, despite another engine being specified in the `Queries.json` file

**How to reproduce**
- Setup at least two engines
- Have an existing project with queries
- Assign an engine to at least one query and save it
- Set the engine NOT assigned to the query as the default
- Reload the project

The query is now assigned the new default engine instead of the one specified in the JSON file

_Originally posted by @Nielswps in https://github.com/Ecdar/Ecdar-GUI/issues/136#issuecomment-1451715069_