SDL or GLFW example

An example based on SDL or GLFW windowing API is needed. As a result, a documentation page with copy&paste code should be created. This task mainly targets homebrew game engines, and would be a reference implementation for future work towards Unity and/or other game engines.

`AWindow` is AUI's built in native windowing API, which is out of the scope of this example. Instead, the `AGLEmbedAuiWrap` should be used and improved if necessary.