Added new engine interfaces, some implementations

- IObject - core interface
- IPositionable, IMovable - position and moving, implementation -
RotatingObject
- IFindable - finding object by point, no implementations
- IDrawable - drawable entity, three implementations: FilledTriangle,
FilledRect, TextureRect
- two collections: ObjectsCollection (all objects are active),
ObjectsSelector (only selected object is active)
- some more stuff