Refactor: SimpleApplication optimization + javadoc

[capdevon]

This pull request introduces significant improvements to the SimpleApplication base class. The primary focus is on enhancing the Javadoc documentation across the entire class, providing clearer explanations for its purpose, default AppStates, key bindings, and the roles of rootNode and guiNode.

Key changes include:

• Comprehensive Javadoc: All fields and methods, now have detailed and accurate Javadoc comments, explaining their parameters, return values, and behavior.

• Clarity and Maintainability: The enhanced documentation and subtle code improvements contribute to better code clarity, making it easier for developers to understand and extend SimpleApplication.

[yaRnMcDonuts]

Looks good to me. I don't see any risks after reviewing everything, but I'm going to leave this one open for a bit longer and do another look through right before merging, just because it is related to such an important core class.