This will make writing libadwaita-based code a bit
nicer, and make it nicer when we use crates that
libadwaita's API re-exports.
Also clean up the unused imports exposed by
this change.
The initial commit of gnome-image-viewer.
Things are in an MVP state, with image viewing and
opening from the desktop functional. We also can
open images externally, print them, and set them as
backgrounds.
