tooltip: Fix warning with GtkCellRendererAccel

tooltip: Fix warning with GtkCellRendererAccel

(gnome-shell-extension-prefs:22382): GLib-GObject-WARNING **: invalid cast from 'GtkInvisible' to 'GtkWindow'
(gnome-shell-extension-prefs:22382): Gtk-CRITICAL **: gtk_window_set_transient_for: assertion 'parent == NULL || GTK_IS_WINDOW (parent)' failed

 #1  0x00007fffeeb81a12 in g_log (log_domain=log_domain@entry=0x7fffeeea3464 "GLib-GObject", log_level=log_level@entry=G_LOG_LEVEL_WARNING, format=format@entry=0x7fffeeeaa878 "invalid cast from '%s' to '%s'") at gmessages.c:1079
 #2  0x00007fffeee9c2a9 in g_type_check_instance_cast (type_instance=0xb314c0, iface_type=iface_type@entry=9020048) at gtype.c:4021
 #3  0x00007ffff0a06e25 in gtk_tooltip_set_last_window (tooltip=tooltip@entry=0xbb8c10, window=<optimized out>) at gtktooltip.c:910
 #4  0x00007ffff0a08dfd in _gtk_tooltip_handle_event (event=event@entry=0xbf72c0) at gtktooltip.c:1538
 #5  0x00007ffff090d7a9 in gtk_main_do_event (event=0xbf72c0) at gtkmain.c:1785
 #6  0x00007ffff0496ab2 in gdk_event_source_dispatch (source=<optimized out>, callback=<optimized out>, user_data=<optimized out>) at gdkeventsource.c:364
 #7  0x00007fffeeb7a8c3 in g_main_dispatch (context=0x807300) at gmain.c:3067

https://bugzilla.gnome.org/show_bug.cgi?id=736131