threadprotect: Remove timeout when quitting.

This prevents blocks from being called that may try to access
destroyed glib objects, and in turn causing an exception.
animatedgif: Replaced the Monitor with a Mutex and ConditionVariable.

Use image height and width for the first frame.
imagebox: Make update_title thread safe.
statusbar: Remove unneeded require.
imagelist: Rewrote the update_cache method.

Caching is more intellegent now.  Also changed local images
so they are loaded with a PixbufLoader.  This makes caching actually
worth it - as Pixbuf.new seems to block the gvl.
imagebox: Prevent crash when freeing gif.