HexCell - Wall joiners

I liked the [HexCell Modular Display Stand System by GillWinkle](https://www.thingiverse.com/thing:4540532), but the collection of designs was missing a piece to fix the stands in the wall (probably, it wasn't designed originally for the wall).

Luckily, the collection included freecad files, and I only added a couple of improvements to make it possible to hang it on the wall with a screw. I have also included the design files in case of the screw hole doesn't fit your needs.