Renamed "CSS" to "Style" as discussed with void256

From the commit message:

> This renames the CSS related part of the system to be known as "styles". Since its not a proper CSS file as a browser would use it, it's a nifty-specific CSS system.
> 
> The original files regarding default CSS handlers and systems have been left untouched (like the parsers and external libraries).
> 
> Javadocs have been changed accordingly as well.

_Discussion in context: http://void256.github.io/nifty-gui/blog/update/nifty-2.0-css/#comment-1760677275_