Categorize series of examples and use different CMakeLists

[chadowo]

Having something like this:

.
└── src/
    ├── basic/
    │   ├── CMakeLists.txt
    │   ├── hello-file/
    │   │   └── hello-file.c
    │   │── hello-world/
    │   │   └── hello-world.c
    │   └── ...
    └── sdl2/
        ├── CMakeLists.txt
        ├── sdl-sprites/
        │   └── sdl-sprites.c
        │── sdl-window/
        │   └── sdl-window.c
        └── ...

Would be ideal.

This way certain categories of examples (like SDL2) can have their own requirements, and if the developer doesn't wishes to build that certain category, they can disable it. This also facilitates adding more varied examples.

Plus it aligns with modern usage of CMake (from what I can gather).

To-do:

• Document the process of adding new examples in CONTRIBUTING.md or similar.