Port the list of official examples to Python/libprocessing

The examples/demos that ship with the Processing Development Environment (PDE) are designed to cover the Processing API in a systematic way:

https://github.com/processing/processing-examples

Each sketch focuses on a specific function or feature. That makes them a useful baseline for checking feature parity.

This issue suggests porting the official examples to Python (mewnala).

Note: there is some prior work in Processing.py, but the syntax differs (camelCase), and only a relatively small subset of examples was ported:
https://github.com/jdf/processing.py/tree/master/examples.py

首先查看 processing-examples 中的官方 sketch 以及 Processing.py 的 examples.py 中现有的子集，然后检查 libprocessing 面向 Python 的入口点。在考虑 issue 中所述语法差异的同时，系统地将这些示例移植到 Python；当官方示例集都有对应的 Python 版本时，即视为完成。