gtk: Add suspended window state
This is implemented using a new xdg_toplevel not_shown
state, and is
meant for allowing applications to know when they can stop doing
unnecessary work and thus save power.
Corresponding Wayland protocol proposal: https://gitlab.freedesktop.org/wayland/wayland-protocols/-/merge_requests/201
The "iconic"/"minimized" state in X11 can probably be mapped to this as well, but didn't bother yet.