image: Rename is_animated_gif to animated.

Added #animated?. Call #free! instead of stop_loading
when cancelling the caching of an animated pixbuf.
animatedgif: Created a new base class for animated pixbufs.

The base class will be used for the AnimatedPNG class too.
booru/browserpage: Add a minimum of 4 for the thread pool.

Just incase cache_size is set to 0.
animatedpixbuf: Fix #wait_for.
imagebox: Use shutdown instead of shutdown!.