WaveShare touchscreen holder insert for Ender 3 and RaspberryPi

This is based on the Ender 3 Touchscreen Case
https://www.thingiverse.com/thing:3030160 by
@Tronnic.

I needed to separate my RPi and screen so I
designed this screen holder that you can use as
drop in replacement for the RPi holder from
@Tronnic's design.

## What to print

In case you'd like to save some time, take a
look at `corner_insert.stl`. It's absolute minimum
version that just supports the corners of the screen
and nothing more.

If you'd prefer to protect the back of your screen
as well, go for `case_insert.stl`. This would give
you full enclosure for the screen with a hole for
the 26 pin connector on the back.