Extended spawner with entity and interval.

"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.
Added "internal" property.

If set to 1, entities will be spawned as children of the entity to which
the property belong.