compositor-wayland: use xdg_shell stable instead of v6

Better to excercise the current rather than outdated protocol.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
- fix continued line alignment

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