Stairs: Improve stair and slab rotation on placement

Stairs: Improve stair and slab rotation on placement

Slabs are placed horizontal instead of vertical, even if they are
placed on a wall.
Slabs are rotated automatically if they are placed to another slab,
no matter which material.
Slabs are placed at the lower position if the placer points into
the lower half of the pointed node and to the upper position if
pointed to the upper half.
Stairs are placed normal if the placer points to the lower half of
the pointed node and rotated upside down if pointed to the upper half.