preview sidebar: only load the thumbnails if previews wanted.

The images for the sidebar previews should only be loaded or queued if the user wants those previews visible. Otherwise it just makes more work that, for some reason, is handled in the UI thread.

This commit removes an appreciable bit of lag when navigating around using the cursor keys in folders full of images, even when the xx-large thumbnails have been generated and are in the user's cache.

I think it's the gdk_pixbuf_new_from_file() call in this handler that's actually causing the lag, but for now let's just respect the preference to keep things speedy for users who don't want the new feature (people like me...)