Entity visibility levels

Entities should have different visibility levels. This is to allow for entities that are visible to the simulation, but not to the players.
The default visibility is "public". These are visible to everyone.
Entities that have "private" visibility are only visible to the simulation or to admin clients.

Perhaps we also should have some kind of "protected" visibility to make some entities only visible to their parents. This would allow us to attach meta-entities to players for example (like quests, or buffs). We need to consider how to handle this though, since we then probably would want some other property to mark these entities are non-tangible.