Rewrite entire build process in Python

[cosinekitty]

The current build process for contributors and automated unit tests uses bash scripts for Linux and Mac, and batch files for Windows. This is not ideal because

• I have to maintain them separately.
• The Windows version leaves out some stuff like complete demo testing.

It would be much better to replace the bash script with a Python program, and then adapt it to work across all 3 supported operating systems.

[cosinekitty]

The main complication will be building C/C++ programs in Windows: it is very different from in Linux/Mac. It may be possible to improve things by using cmake.