shell-util: Add helper to hide actors from pick

At times it is desireable to hide actors from being picked even
with a mode of CLUTTER_PICK_ALL.

Currently we use a pattern of
    clutter_actor_hide();
    clutter_stage_get_actor_at_pos();
    clutter_actor_show();
in these cases, which gets hideous if the actor we want to exclude
from the pick is located in another module.

A more elegant solution is to connect a handler to the ::pick signal,
which stops further emission.

Credit for the idea goes to Owen Taylor.

https://bugzilla.gnome.org/show_bug.cgi?id=634560

src/shell-util.h

@@ -4,12 +4,14 @@
 #define __SHELL_UTIL_H__
 
 #include <gio/gio.h>
+#include <clutter/clutter.h>
 
 G_BEGIN_DECLS
 
 char *shell_util_get_label_for_uri (const char *text_uri);
 GIcon *shell_util_get_icon_for_uri (const char *text_uri);
 GIcon *shell_util_icon_from_string (const char *string, GError **error);
+void   shell_util_set_hidden_from_pick (ClutterActor *actor, gboolean hidden);
 
 G_END_DECLS