Use named pathfinding constants.

- Rename CostForInvalidCell to PathCostForInvalidPath
- Add MovementCostForUnreachableCell
- Update usages of int.MaxValue and short.Maxvalue to use named constants where relevant.
- Update costs on ICustomMovementLayer to return short, for consistency with costs from Locomotor.
- Rename some methods to distinguish between path/movement cost.

OpenRA.Mods.Common/TraitsInterfaces.cs

@@ -404,8 +404,8 @@ namespace OpenRA.Mods.Common.Traits
 		bool ReturnToGroundLayerOnIdle { get; }
 
 		bool EnabledForLocomotor(LocomotorInfo li);
-		int EntryMovementCost(LocomotorInfo li, CPos cell);
-		int ExitMovementCost(LocomotorInfo li, CPos cell);
+		short EntryMovementCost(LocomotorInfo li, CPos cell);
+		short ExitMovementCost(LocomotorInfo li, CPos cell);
 
 		byte GetTerrainIndex(CPos cell);
 		WPos CenterOfCell(CPos cell);