ivi-shell: introduce ivi-layout-shell.h

This new header encapsulates the API that ivi-layout offers to
ivi-shell.c to call.

ivi-shell.c no longer uses ivi-layout-private.h. This limits the
ivi-layout internal structures to just ivi-layout code.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Emre Ucan <eucan@de.adit-jv.com>