add base struct for all game objects

Add GameObjecter interface and GameObject struct implementing it. Use it
for base such classes as ActiveObject and Item (in future).
fix GetInfo methods
add base for inventory and items
significant changes in GameField structure

Create specialized fieldCell struct. Now map is a two-dimensional array of
ones. Each cell contain three layers: background, actors and items in
cell. Add specialized API for map.
drop items after mob's death