XDG-Shell: update to 1.6.0

Allow the OZONE_WAYLAND_USE_XDG_SHELL environment variable
to work with the latest Weston 1.6.0 release. It will not
work with Weston 1.5.0 anymore. Functionality is unchanged.

(we define a general "wl_array_for_each_type()" macro to
handle "wl_array" types nicely in the future)