port the clients to xdg-shell stable

A few remarks:

• I tested most of the examples against gnome-shell.
• Since I'm not familiar with autotools I only changed the Meson bits.
• I'm not familiar with weston. This was mostly a simple find and replace of zxdg_shell_v6 to xdg_wm_base, zxdg_toplevel_v6 to xdg_toplevel and zxdg_surface_v6 to xdg_surface so please review carefully.

closes #119 (closed)