BIQU PITFT50 v2.0 touch screen case

A remix of [thing 4689019](https://www.thingiverse.com/thing:4689019) to make it fit on my particular make of the BTT PI TFT50 v2.0 touch screen by BIQU. The original design did not fit well, some resistors are in the way of the rear plate. Neither the original v2 update nor the other v2 remix available at the time of writing work with my screen.

Also added some more changes to the rear Pi case:
- Made it compatible with my Pi 3B+ (should remain compatible with Pi 4 but I don't own one to check it).
- Added an opening for the GPIO connector, in order to facilitate the occasional need to connect an ADXL sensor for Klipper's Input Shaping.
- Replaced the ventilation grid on the side with a wide opening, which if need arises allows swapping SD cards without having to remove the case.
- Removed the printed mounting posts to use the metal ones and the set of screws that came with the display instead, but there's a separate STL with these if you still prefer that (requires 4 x M2.5x25