Better Case for Raspberry Pi Zero + TV HAT

I modified Radobyl's model, making the following changes:

* I made four holes in the case part for M2.5 screws, so the board can be fixed into place.
* I modified the lid, so it does not have hole for GPIO ports, as they won't be used in my case.
* I designed new part `RaspiTV_Case_bottom.stl`, which contains "legs" that hide the four screws used