animtedgif/statusbar: Rename lock class variables to mutex.
imagebox: Prevent crash when next/previous_image.
ahoviewer: Replaced thread gem dependency.

Tweaked many things that use the ThreadPool class.
Fixed crash when freeing the currently displayed gif.
Added animation frame preloading to create pixbufs prior
to trying to display them.

Worked out a lot of bugs when saving images from a booru.