xdg-shell: Introduce xdg_positioner

xdg_positioner is a method for declarative positioning of child surfaces
(currently only xdg_popup surfaces). A client creates a description of a
positioning logic using the xdg_positioner interface. The xdg_positioner
object is then used when creating a xdg_popup for describing how the
child surface should be positioned in relation to the parent surface.

Signed-off-by: Jonas Ådahl <jadahl@gmail.com>
Signed-off-by: Mike Blumenkrantz <zmike@samsung.com>
Acked-by: Yong Bakos <ybakos@humanoriented.com>
Acked-by: Quentin Glidic <sardemff7+git@sardemff7.net>
Reviewed-by: Bryce Harrington <bryce@osg.samsung.com>