[ShellGenericContainer] Add method to get number of skipped children

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https://bugzilla.gnome.org/show_bug.cgi?id=610740

src/shell-generic-container.h

@@ -42,6 +42,8 @@ struct _ShellGenericContainerClass
 
 GType shell_generic_container_get_type (void) G_GNUC_CONST;
 
+guint shell_generic_container_get_n_skip_paint (ShellGenericContainer  *container);
+
 void shell_generic_container_set_skip_paint (ShellGenericContainer  *container,
                                              ClutterActor           *actor,
                                              gboolean                skip);