xdg-shell: Turn xdg_surface into a generic base interface

Split out toplevel window like requests and events into a new interface
called xdg_toplevel, and turn xdg_surface into a generic base interface
which others extends.

xdg_popup is changed to extend the xdg_surface.

The configure event in xdg_surface was split up making
xdg_surface.configure an event only carrying the serial number, while a
new xdg_toplevel.configure event carries the other data previously sent
via xdg_surface.configure. xdg_toplevel.configure is made to extend,
via the latch-state mechanism, xdg_surface.configure and depends on
that event to synchronize state.

Other future xdg_surface based extensions are meant to also extend
xdg_surface.configure for relevant window type dependend state
synchronization.

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