replaced deprecated functions register_sidebar_widget nd register_widget_control

I made some very minor changes to the plugin. When debugging some other stuff I noticed the plugin uses the functions register_sidebar_widget and register_widget_control. They are deprecated since WordPress 2.8. I replaced the functions with wp_register_sidebar_widget and wp_register_widget_control.

https://www.ads-software.com/plugins/collapsible-archive-widget/