Allow suspension of individual entities.

When the "suspended" property is applied on individual entities, the
Tick operation will be intercepted for the specific entity and ignored.
If the property is applied to the world entity _all_ Tick operations
will however be suspended.
Suspend entities placed in limbo.

And resume when they are moved out of limbo. This fixes an issue where
entities in limbo kept on being destroyed over and over again, producing
an ever increasing number of skulls (due to the "decays" property).
Added integration test for suspended property.