Adding a world structure to the same place twice should result in moving it to the end of the vector

[TheRealKaup]

Currently, adder functions will return false if a world structure is already stored in the vector, and do nothing else.

Instead, they should also move it from its current position to the end of the vector, since it's likely the intuitive intention of users who accidentally added a world structure twice to the same place (whether they forgot they added it the first time or not).