[Sub-Issue] Implement FlixelButton and FlixelFAB

[stringdotjar]

Create a standard button and a circular "Floating Action Button" (FAB).

Requirements (Standard):

• Support for 3-state frame logic (Normal, Highlight, Pressed).
• Automatic label centering.

Requirements (FAB):

• Circular bounding box for collision/interaction.
• Support for a centered icon sprite.