Solve issues in method `addNewAgentToScenario`

https://gitlab.lrz.de/vadere/vadere/blob/master/VadereSimulator/src/org/vadere/simulator/control/SourceController.java#L179

• Copied from method: TODO [priority=high] [task=refactoring] this is bad programming. Why is the target list added later? What if Pedestrian does something with the target list before it is stored?

• Copied from method: TODO [priority=high] [task=refactoring] why only if he has no targets? because the createElement method might add some.

• Method returns newElement but it is actually never used, so void should do as well.

• [ ] IntelliJ suggests to turn the method private (before check if plausible)

• IntelliJ suggests to turn the method package-private (before check if plausible)