Implement suspended xdg_toplevel state

When a window is on a different workspace, minimized, obscured etc, right now the client isn't really aware that it is. If it would, it could do some power saving. Web browsers can (and do) for example slow down the Javascript code etc.

This implements support for the not_shown toplevel state proposed for xdg-shell: https://gitlab.freedesktop.org/wayland/wayland-protocols/-/merge_requests/201, which is meant for this use case.