st/image-content: Implement GIcon interface

st/image-content: Implement GIcon interface

On the one hand, this is a bit of a stretch: StImageContent is what
we create from GIcons.

But on the other hand, there's some justification: StImageContent does
represent an image (and likely icon) after all, and there's some
precedent with GdkPixbuf.

In the end as we don't care about serialization or loading from other
API, we can go with a very crude implementation that allows us to
pass out a content as GIcon and use it directly when "loading" it.

We will use that soon to represent X11 window icons as GIcons, which
in turn will allow us to unify app icon handling.

https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1342