I must create a plug-for the reason that adds a widget in wordpress.

I'd like the widget to become in line with the current pages widget but add a chance to choose either include OR exclude by drop lower or checkbox then list pages by ID.

Following this guide I could alter the exclude to incorporate by hacking the core file situated in wordpress-includes/default_icons.php.

This is actually the original default_icons.php:

http://pastebin.com/Rg2vTMmK

How do you result in the changes and transform it into a wordpress plugin?

Thank You