xwayland/window-manager: Add support for _NET_FRAME_EXTENTS

https://specifications.freedesktop.org/wm-spec/1.4/ar01s05.html

 says
"The Window Manager MUST set _NET_FRAME_EXTENTS to the extents of the
window's frame", so this is probably something we should be doing.

Some programs (such as some versions of Firefox) expect this to be present,
and will render popups in wrong locations if it's not.

Signed-off-by: Derek Foreman <derek.foreman@collabora.com>