libweston-desktop: implement the new xdg_shell (stable) protocol
Some clients like the mpv video player now request the xdg_shell protocol so these will fail if the compositor only provides the xdg_shell_unstable_v6 protocol. Compositors like mir and gnome provide both protocols. The two protocols are very similar therefore the code in xdg-shell-v6.c has been refactored to work with the new xdg_shell protocol and now resides in xdg-shell.c. All the sample programs including compositor-wayland.c have been changed to use the new protocol.