Refactored [sub-]cell availability logic in IPositionable & ActorMap

Moved the logic from IPositionable.CanEnterCell & integrated sub-cell selection.
Added IPositionable.IsMovingFrom(CPos location, int subCell = -1) - to detect transient actors
Renamed IPositionable.{GetDesiredSubcell => GetAvailableSubcell} - since it checks for available sub-cells
Reduced IPositionable.CanEnterCell to one method that usually uses IPositionable.GetAvailableSubcell
Added actor checking to ActorMap.{HasFreeSubCell, FreeSubCell, AnyUnitsAt} - used by [sub-]cell availability logic