refactor EntitySystem proxy into its own class, give proxy to BUIs

[deltanedas]

• replaced EntitySystem.Proxy.cs with abstract class EntManProxy, and moved RaiseLocalEvent etc to it as well
• this is inherited by EntitySystem and BoundUserInterface, you can now do TryComp and stuff in BUIs
• PlayerManager is also protected so systems can stop adding it as dependency
• anything in content can use it too with IoCManager.InjectDependencies(this); and it just works, as long as there is no other parent class. BUI and EntitySystem had no base class so it works for them

no breaking changes, BUI still has EntMan to get the entity manager so code doesnt need changing