Implement clone-restore interface in ObjectCollection (+ tests)
Emit "beforeAdd/Set/Remove" events in ObjectsCollection (+ tests)

+ fix .set() method.
Refactor electric fields, use ObjectsCollection (step 1)

Engine is refactored completely, but also modeler can be optimized,
as we still have repetitive set / getElectricFieldProperties etc.
Implement units translation support in objects collection (+ tests)
Dispatch "beforeChange" and "change" events in ObjectsCollection