[Parent] Implement core UI component library

[stringdotjar]

This is the master issue for developing a comprehensive UI library for FlixelGDX. The goal is to provide a set of standardized, highly customizable components that follow the Flixel philosophy while leveraging libGDX's efficiency.

Core Requirements for All Sub-Issues:

• Must extend FlixelSpriteGroup or a common FlixelUI base.
• Must support interaction signals (onOver, onOut, onClick, etc).
• Must be "Camera-Aware" (staying on a UI-dedicated camera by default).
• Using interfaces where necessary.