image: Removed useless return value from set_pixbuf.
booru/site: Minor changes to tags code.
animatedgif: File.exists? -> File.file?.
ui: Added quit menuitem to main context menu.
statusbar: Remove main_iteration call in set_message.
image: Use !! in Image::valid?.
imagebox: Call cancel_draw when ThumbnailBar selection changes.

This is a better fix than 024cb67dfd1074c5d4bce78446af345588edfc3b.
imagelist: Use a thread pool for caching images.
imagebox: Allow panning with middle mouse click.
imagebox: Remove explicit interpolation type in scale calls.

This was initially going to be configurable by the user, but
I no longer think there is a point for it to be.
ui: Better setting of sensitivity for certain actions.
imagebox/imagelist: Improved navigation methods for local list.
imagebox: Use a thread instead of GLib::Timeout for the slideshow.

Round zoom percent in #zoom, and synchronize #set_active_image_list
calls.
booru: Rewrote a large portion of the download code.

Now using httpclient instead of net/http. The network performance
increase is quite noticable.
booru: Rename booruimage.rb to image.rb