xdg-shell: Add toplevel "bounds" configure event

This aims to communicate the maximum size a surface should be created
with, and loosely corresponds to the concept of "work area" in the X11
world.

Closes: wayland/wayland-protocols#17

Signed-off-by: Jonas Ådahl <jadahl@gmail.com>
Reviewed-by: Simon Ser <contact@emersion.fr>
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