Implement a service provider/locator class

[jstamant]

Consider implementing a service, service provider, and service locator class.

Right now, game entities occasionally refer to the window and mouse for performing logic. The EntityManager is providing these references upon entity creation. Perhaps the service pattern may help decoupling.

Refer to http://gameprogrammingpatterns.com/service-locator.html for further information regarding the service pattern.