Update xdg-shell for Weston 1.5

The protocol has heavily changed, update it so it builds and works
with the newest Weston 1.5 release.

Add some README instructions.

XWALK-1382

Signed-off-by: Manuel Bachmann <manuel.bachmann@open.eurogiciel.org>
I'm not sure your patch works in face of this:
https://github.com/01org/ozone-wayland/commit/e57ab035f78d82117e1f1c7ec1459eb0c21deb22

have you tested?