Allow to specify subunits on creation.
This basically does two things:
- Allow turrets on moving entities (fixes serialisation in cmpPos).
- Allow an entity to be present on another when the other is created.
That makes it basically a boiled down version of a prior patch, omitting the part where orders could be passed on.
This does allow e.g. ranged units on chariots that fire while the chariot moves.
Original diff by: @sanderd17
Redone by: @Stan
Differential revision: D1958
Comments by: @Alexandermb, @Angen, @bb, @Langbart, @Nescio, @Stan, @wraitii
Refs. #2577 by implementing the entity on platform case.