threadpool: Fix resizing.
mainwindow: Only parse_geometry if x and y are set.
statusbar: Add critical priority for menuitem tooltips.

Also clear progress bar priority in #clear_progress.
booru/browserpage: Fix crash when only 1 post is retrieved.
booru/image: Change parameter order in constructor.
statusbar: Remove unneeded show_all call.
imagebox: Fix scrollbar hiding.
booru/site: Remove retry on 404 and 503.

This was a bad solution to a problem that is not mine.
Gelbooru, maybe others, adds posts to their api post index
before the images have been moved to where they say they are.
threadpool: Fixed deadlock when trying to join dead threads.

Also changed #shutdown to finish all queued tasks, not just
the running ones. Moved around the preload threadpool in
the imagebox.
image: Made local thumbnails smaller.

Also added a border, and a checkered background for
transparent images (only for the thumbnails).