Plan furniture layout in 2D, not 3D, for easier snapping

Plan furniture layout in the 2D space instead of the 3D space so snapping works and furniture stays where you put it.

1. Furniture layout should be planned in the 2D space rather than the 3D space, because snapping in 3D is difficult and furniture can drift easily.

2. In 2D the movement of furniture is easier to control.