xdg-shell: Rename interfaces

Rename the interfaces according to the wayland-protocols policy. Since
the name 'xdg_shell' as an interface was already taken (by
xdg-shell-unstable-v5) zxdg_shell_v6 was renamed xdg_wm_base. The
surface role related interfaces were not renamed, as naming collision
is only unmanagable when exposed as globals via the registry.

Signed-off-by: Jonas Ådahl <jadahl@gmail.com>
Reviewed-By: Mike Blumenkrantz <zmike@osg.samsung.com>