wayland/xdg-shell: Port to unstable v6

Port the xdg_shell implementation to use the unstable v6 protocol. This
includes:

 - making xdg_surface a generic base interface for xdg_shell surface
   roles
 - create a xdg_toplevel role replacing the old xdg_surface
 - change the xdg_opup role to be based on xdg_surface
 - make xdg_popup not grab by default
 - add support for xdg_positioner

https://bugzilla.gnome.org/show_bug.cgi?id=769936