MAP: Use standard smart pointers to manage memory allocation of game objects

[tturocy]

For very historical reasons, our handling of referring to member objects of games (via the GameObject and GameObjectPtr system) conflates two logically distinct considerations:

• The validity of an object (that is, whether it continues to refer to a legitimate part of the game, or has been deleted at some point by some operation);
• The reference counting of pointers to an object (to ensure memory gets deallocated correctly).

This leads to object deletion at potentially surprising times, because in GameObject there is a place where delete this is called - which is not ideal, even if it is technically legal. Without being exceptionally careful one can easily enough create a situation where an operation is attempted on an object that was deleted as part of the Invalidate() call on it.

• Use std::shared_ptr and std::weak_ptr as appropriate to handle memory management of game component objects
• Create a new variation on GameObjectPtr which stores a std::shared_ptr to the object, and checks for validity, but which does not have any responsibility for reference counting.