"entity" allows one to better specify how the new entity should be
created. It's optional; if omitted a default entity of the type in "m_type" is
created.
"interval" allows for specification of the interval between ticks. It's
optional.
When an entity has this property it acts a an spawner entity. At an
interval it checks if there are enough of the specified entities
available, and if not it spawn new ones. This allows for some better
gameplay mechanics.
