Add custom panels for non-standard rails and supports

Add, copy and shape custom panels inside a cabinet to place rails and supports where the standard structure provides none.

1. Custom panels can be added, moved, rotated, stretched and copied with specific dimensions and distances.

2. Multiple copies can be created at fixed intervals to position back rails at custom proportions.

3. A custom panel added inside a cabinet can be given its own core material and finish and a custom label describing its purpose (for example a top rail).

4. A custom panel added inside a cabinet can be placed where the standard cabinet structure provides none, so non-standard support panels can be placed where the standard cabinet structure provides none.

   Stability rails are now positioned from the side, bottom and top panels of the cabinet rather than from the back panel: they span the full internal width and are evenly spaced between the bottom and top panels. Cabinets created in older versions keep their original rail placement.