wayland: Add api to set a foreign shell

Since the wayland protocol doesn't currently provide a way to
retrospectively query the interfaces that get notified when a client
first connects then when using a foreign display with Cogl then we also
need api for telling cogl what compositor and shell objects to use. We
already had api for setting a foreign compositor so this patch just adds
api for setting a foreign shell.

This patch also adds documentation for all the wayland specific apis.

Reviewed-by: Neil Roberts <neil@linux.intel.com>

cogl/cogl-renderer-private.h

@@ -56,6 +56,7 @@ struct _CoglRenderer
 #if defined (COGL_HAS_EGL_PLATFORM_WAYLAND_SUPPORT)
   struct wl_display *foreign_wayland_display;
   struct wl_compositor *foreign_wayland_compositor;
+  struct wl_shell *foreign_wayland_shell;
 #endif
   /* List of callback functions that will be given every native event */
   GSList *event_filters;