desktop-shell: Re-position views when outputs change

When the last output is destroyed or when a new output is created after
the last output is destroyed, we need to re-position the views to ensure
that all the views are displayed on the output.

Fixes: https://gitlab.freedesktop.org/wayland/weston/issues/210



Signed-off-by: Harish Krupo <harishkrupo@gmail.com>