game_engine. Update engine create object method to get game data.

Add engine invalid operation error.
Add game data argument to engine create object method.
In ingenium throw engine invalid operation error if parent is null.
game_engine. Add children field to ludus entity.

Add children field.
Add begin/end child method to access a begin/end child iterator.
Add number of children initialization argument.
game_engine. Update project SDK version to Android-18.