Support for configuring minimal cost for moving between tiles

- Added `movementCostBase` parameter to game config that defines minimal
amount of movement points that will be spent when moving from one tile
on another while offroad (and cost of Fly / Town Portal spells)
- Added `BASE_TILE_MOVEMENT_COST` bonus type that allows modifying
`movementCostBase` on per-hero basis

Example usage for hota-like pathfinding skill
```json
"tileCostReduction" : {
	"type" : "BASE_TILE_MOVEMENT_COST",
	"val" : -15
}
```

lib/RoadHandler.cpp

@@ -84,6 +84,6 @@ RoadType::RoadType():
 	id(Road::NO_ROAD),
 	identifier("empty"),
 	modScope("core"),
-	movementCost(GameConstants::BASE_MOVEMENT_COST)
+	movementCost(0)
 {}
 VCMI_LIB_NAMESPACE_END