Stackable pallet for the excellent mini warehouse by @WimV.

Makes it easier to transport multiple empty pallets on the toy fork lift.

Notable features:

• Slots for stacking pallets that also serve as guides holding pallets in place on the beams
• No middle bottom plank for less supports and easier fixing of rubber bands
• Ribs for more rigidity with the same amount of material

Printing:

• Slots for stacking do not require supports
• No need for supports for the middle plank :)
• Prints fine with both 0.4 (tested @0.2 layer height) and 0.6 (tested @0.3 layer height) nozzles