Undo does not cancel parenting of subemmiters.

Repro steps:
1. In the scene hierarchy create two particle systems A & B.
2. Select A and enable subemitters on it.
3. Add B as one of A's subemitters.
4. Upon being prompted whether to reparent, click yes.
5. B is moved to a be a child if A in the hierarchy.
6. Press Ctrl+Z - In hierarchy view B still stands under the A.

Expected outcome: Undoing to undo reparenting of particle systems.

Note:
- not a regression, happens since early 4.x
- still present in latest 5.4.0b10