World: GetUnitAt() and NearestUnitOf() becomes const methods
WalkTile: first implementation
World: Slowly moving to walkable tile from buildtile
Path: Computation done on WalkTile basis
Path: Kill old BuildTile dead code + update DumpPath_() for WalkTile
Debug: Only display walkabletiles when movable unit is selected
Path: early stop in shortest path computation if destination isn't reachable
GatherOrder: Reset move order target pos only if target unit has changed
UnitDesc: Unit dimensions are multiple of WALK_TILE_SIZE
World: NearestWalkableTileOf() first implementation using BFS
Path: If the destinatino isn't walkable, find a suitable alternative goal
Path: Uses a priority queue for shortest path computation
Path: Diagonal move are allowed in pathfinding