Add custom font loading demo and PostBufferPaint background animation

Custom Font Loading Window:
- Added 3 fancy fonts to Examples/FigleShowcaseExample/Fonts/
  * cyberlarge.flf - futuristic style
  * isometric1.flf - 3D isometric look
  * starwars.flf - Star Wars style
- Created CustomFontsWindow demonstrating FontPath property
- Shows how to load external .flf font files

Rainbow Window Enhancement:
- Added PostBufferPaint hook for animated wave background
- Creates colorful wave pattern using Math.Sin
- Wave animates with _waveOffset variable
- Background uses darkened rainbow colors
- Updates every 50ms for smooth animation
- Demonstrates compositor-style buffer manipulation

Menu Updates:
- Added button 3: Custom Font Loading
- Renumbered rainbow to button 4
- Updated descriptions to mention PostBufferPaint
- Updated keyboard shortcuts (1-4)
- Added custom fonts to feature list

Author: nickprotop