xdg-shell: make it also conditionnal at runtime

Using env variable : OZONE_WAYLAND_USE_XDG_SHELL
(name is inspired from Qt and EFL implementations of xdg-shell)

So far xdg-shell is an extension to weston, so let's make it available at runtime
without this env variable defined (ie: in /etc/profile),
at least wl-shell will be used as (mandatory) fallback.

Signed-off-by: Philippe Coval <philippe.coval@open.eurogiciel.org>