clients: use xdg_shell stable instead of v6

Now that Weston supports the stable revision, use it. Better to excercise the
current rather than outdated protocol.

Pekka:
- split the patch, rewrote commit message
- rename xdg_shell_ping to xdg_wm_base_ping
- rename xdg_shell_listener to wm_base_listener
- rename shell to wm_base
- fix continued line alignment
- drop unrelated change of adding parentheses around bit-wise and

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>