Add backplate thickness customization to Underware_Item_Holder.scad

[mnivoliez]

Add backplate thickness customization

[SnazzyGreenWarrior]

1. Looks like this conflicts with the changes here
2. 4.8 and 6.5 are magic numbers. While I generally don't like magic numbers, the backplate generation logic would need to be reworked to support attaching outward instead of the cutting inward approach currently used. To validate this, override Multipoint connection's 4.8 with value 6.5 and attempt a render. In my test, the channel is buried in the backplate and is unusable. This means that users would need to know the magic numbers and set them to the proper one before render.