ivi-shell: avoid inserting a ivi_layer to multiple screens.

ivi-shell: avoid inserting a ivi_layer to multiple screens.

In just previous wl_list_for_each_safe, list of layer in a screen is
cleaned up. And then, the list is re-constructed from
pending.layer_list.

In this re-construction, if order.link of a layer were inserted into a
screen whose number is later one of current screen, the order.link will
inserted into layer_list of two screens. This shall be avoided.

However, if we want to implement a feature to allow a layer to be added
to multiple screens. A layer shall have several order.link per screens.
So, I marked here as TODO.

Signed-off-by: Nobuhiko Tanibata <NOBUHIKO_TANIBATA@xddp.denso.co.jp>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>