Introduce GBytesIcon

GBytesIcon is an icon that has a GBytes inside of it where the GBytes
contains some sort of encoded image in a widely-recognised file format.
Ideally this will be a PNG.

It implements GLoadableIcon, so GTK will already understand how to use
it, but we will add another patch there to make things more efficient.

https://bugzilla.gnome.org/show_bug.cgi?id=688820