added ActionPane, InfoPane, and InventoryPane

haven’t done anything with NelsonGame class yet. Just a copy of
MikeGame.
Made GridMap Serializable so we can later use it to load maps.
Added with and height fields and accessors.
Added a default constructor that initialises this to (0, 0).
Added methods for removing entities and tiles from the map.
Made this implement Serializable for map loading purposes.
Added the map builder class.