• Switching to a window that is not responding no longer freezes CKFlip3D.
  The switch is refused instead: the cascade stays open, the front tile
  shakes and a notice explains why. Another window or cancel works as usual
• Only responsiveness is checked, never the preview, so fullscreen games and
  windows with an empty preview still switch normally
• A hung window with no live preview is no longer captured the slow way,
  which could freeze the cascade while it opened
• New Force quit (Controls -> Mouse & keyboard): End pressed five times in a
  row quits CKFlip3D even when it has stopped responding, restoring hidden
  windows and the taskbar first. Key, mouse button and press count (x1-x5)
  are configurable; empty = off
• New Cancel clears search first (Search, default on). Off = the cancel key
  closes the cascade right away, even with a query typed