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>
You're right, I haven't tested (I build ozone via crosswalk). It will obviously make XDG build but fail at runtime.  The solution would be to have a "use_wayland_version='1.5.0'", which would be required along with "use_xdg_shell". Will see that.