menubutton: Remove prelight state when button is deactivated

If the button is deactivated, it should not appear to be hovered
anymore.

https://bugzilla.gnome.org/show_bug.cgi?id=781992

gtk/gtkmenubutton.c

@@ -652,6 +652,7 @@ static gboolean
 menu_deactivate_cb (GtkMenuButton *menu_button)
 {
   gtk_toggle_button_set_active (GTK_TOGGLE_BUTTON (menu_button), FALSE);
+  gtk_widget_unset_state_flags (GTK_WIDGET (menu_button), GTK_STATE_FLAG_PRELIGHT);
 
   return TRUE;
 }