Shell: Prepare to support xdg_shell.

We want to easily add support for different shells supporting Wayland
protocol(i.e. wl_shell, xdg_shell etc). This CL adds the necessary
interface in WaylandShellSurface and wl_shell support is implemented
in WLShellSurface. xdg_shell support will be added later.

Origin: https://github.com/01org/ozone-wayland/commit/8261030a07a3db189aee360a8fc0a2187e68cd94

Change-Id: I0fba1e01f0e1c6f7162672930bd4f28713beac6f
Author:    kalyan kondapally <kalyan.kondapally@intel.com>
Signed-off-by: Manuel Bachmann <manuel.bachmann@open.eurogiciel.org>