ivi-shell: add exit binding

Let's me easily exit ivi-shell when testing with the DRM-backend.
Only available in ivi-shell developer mode.

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