[Sub-Issue] Implement FlixelPanel (9-Slice backgrounds)

[stringdotjar]

Create a FlixelPanel class that serves as the base container for other UI elements.

Requirements:

• Support for 9-slice scaling using libGDX NinePatch.
• Adjustable border thickness and background tinting.
• Automatic "Child Clipping" (togglable) to ensure contents don't spill outside the panel boundaries.