Extend game::handle for other use-cases

[Qartar]

game::handle<T> is currently hard-coded to work with game::world and game::object. It would be convenient if this worked with other systems that would benefit from handles such as sound assets, sound emitters, render models, fonts, etc.

This would likely require implementing a generic system class to replace the interactions with game::world and a generic methods to replace conversions to game::object.