Rename the dark tile to `slow`

No need to have an unbreakable tile since the void will kill players (as
soon as it is implemented).

Resources/TileSetClassic.tres

@@ -54,7 +54,7 @@ points = PoolVector2Array( 0, 0, 64, 0, 64, 64, 0, 64 )
 "shape_transform": Transform2D( 1, 0, 0, 1, 0, 0 )
 } ]
 2/z_index = 0
-3/name = "unbreakable"
+3/name = "slow"
 3/texture = ExtResource( 1 )
 3/tex_offset = Vector2( 0, 0 )
 3/modulate = Color( 1, 1, 1, 1 )