This attempts to resolve issues #753 (closed) and #3316 (closed).
Currently, where a GIF is overwritten as an animation or static image depends on the previous configuration values (whether you checked "Save as animation" the last time you exported). This means that you can lose frames when overwriting a GIF if you previously exported a static image.
This patch adds a parasite to an animated GIF on import. When exporting, we then check if that parasite exists. If it does, we set the animation configurations appropriately and then detach the parasite so it doesn't impact future exports.