* prevent public classes from being created on the stack
- we used to to this by deleting operator delete, but this prevented
the internal "F" classes from being virtual; which can be useful
when using EntityManger::Listener.
now we just make the destructor protected in each class.
- EntityManger::Listener now has a virtual destructor so that
objects could be correctly destroyed from Listener*
* improve EntityManger and Component managers
- all component managers now have the same "base" API
- getComponentCount()
- empty()
- getEntity()
- getEntities()
- Scene now has getEntityCount()
- EntityManager now has getEntityCount()
- all component manager implement gc() the same way, by calling destroy()
- SingleInstanceComponentManager::gc() that calls removeComponent() has
been removed because it's dangerous. removeComponent() is often
not enough, some additional cleanup might be needed.
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