Is self created for internal purpose?

Fictive entities are used internally but they should not be exposed to the final user.

Property definitions

nitc $ MEntity :: _is_fictive
	# Is `self` created for internal purpose?
	#
	# Fictive entities are used internally but they should not be
	# exposed to the final user.
	var is_fictive: Bool = false is writable
src/model/model_base.nit:101,2--105,41