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

OpenRA.Mods.RA/Activities/UnloadCargo.cs

@@ -38,7 +38,7 @@ namespace OpenRA.Mods.RA.Activities
 
 			return cargo.CurrentAdjacentCells
 				.Shuffle(self.World.SharedRandom)
-				.Select(c => Pair.New(c, pos.GetDesiredSubcell(c, null)))
+				.Select(c => Pair.New(c, pos.GetAvailableSubcell(c, -1, null)))
 				.Cast<Pair<CPos, int>?>()
 				.FirstOrDefault(s => s.Value.Second >= 0);
 		}