Store a pointer to neighboring tiles in mapNodes

[SimplyLiz]

Instead of providing a function to get a mapNodes neighbors in the map class, the (3x3) neighbors could be stored as pointers in the mapNode itself to avoid having to call the function every time neighbors need to be accessed.

It's probably not worth the effort performance wise, but it'd be cleaner this way and the responsiblity of tracking it's neighbors would go to the mapNode class.

[SimplyLiz]

Discarded. After discussion it turned out to not make much sense to change this.