Add a weston.ini option for ivi-shell to enable "developer mode".

When developer mode is enabled, hook up the debug key bindings.

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>