Add GIF support

[marcinjahn]

Some extensions on extensions.gnome.org use gif as the extension image.
Extensions Manager does not display gif animations, it just displays the first frame of the gif.

Here's some example of an extension with GIF image: https://extensions.gnome.org/extension/3438/jiggle/

[mjakeman]

Hi, I looked into this at one point (I don't believe there was ever an issue).

It seemed quite hard to do at the time, which is why I never got around to it.

I think we could use these:

• https://docs.gtk.org/gdk-pixbuf/class.PixbufAnimation.html
• https://gist.github.com/mertyildiran/f2a0746fbdbce728911854a2f038ee22

[mjakeman]

There's an example from gtk-rs here too:

• https://github.com/gtk-rs/gtk4-rs/tree/master/examples/gif_paintable