ivi-shell: add API for weston_surface -> ivi_layout_surface

Add ivi-layout API for getting an ivi_layout_surface from a
weston_surface if it exists. This can be used by controllers that hook
up to core Weston callbacks and get handed a weston_surface, but need to
use ivi-layout API to manipulate it.

The only ways ivi-layout itself would be able to go from weston_surface
to ivi_layout_surface are either searching through the list of all
ivi_layout_surfaces or adding a dummy destroy listener to the
weston_surface. Therefore the implementation is delegated to
ivi-shell.c.

Ivi-shell.c can easily look up the ivi_shell_surface for a
weston_surface, and that will map 1:1 to an ivi_layout_surface.

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