Simple frame support for Raspberry Pi Touch Display 2

Based on the display frame for the earlier [Raspberry Pi Touch Display](https://www.thingiverse.com/thing:2756684) by [Plasticore](https://www.thingiverse.com/Plasticore/designs), but re-drawn in Freecad so I could adapt the dimensions for the newer "2" version. (The original was curvier and prettier!)

I used the excellent [model of the Raspberry Pi Touch Display 2](https://www.printables.com/model/1067072-official-raspberry-pi-touch-display-2-reference-mo/files) by [@Rojikku_56138](https://www.printables.com/@Rojikku_56138) as a shapebinder in Freecad.

There are a number of improvements I'd like to add if I ever need to print a 2nd one of these:

* more chamfers around the edges
* set the screen in a little deeper to better protect its edge
* reduce the screw hole size and add a pocket for the heads (it does however work as is with the screws that come with the display)

Unlike the original this is really intended as an open frame and does not cover the raspberry pi at all.