libweston-desktop: implement the new xdg_shell (stable) protocol, take 2

This is a polished version of !62 (closed). Weston should be entering the major feature freeze soon with an alpha release, and the original author does not seem to be available at the moment. !62 (closed) also didn't allow maintainer pushes, so I cannot just push a new revision there, I needed to open a new MR.

This is the original description:

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.

Anthony Enrione ant8me@gmail.com