how to create a custom widget that uses the internal templat

We are working on a project and want to create some custom widgets that leverage the internal Widget Templates feature instead of using external files.   I am having a hard time finding current documentation on how best to do this.   How do we add our own custom types of widgets (“applies to” in the dropdown), and how do we code the custom widget to pull from that resource when being setup and displayed?

I see lots of examples and documentation about changing the widgets (both built in and custom) to use EXTERNAL templates, but not much on how to leverage the internal widget templates.  

I have spent a few hours looking through blogs and forum posts, and figured at this point I might as well ask.  

Hi Jason,

I have a project on GitHub which I used for my presentation during the FalafelCon 2013 conference. I wrote a blog about it which contains a link to GitHub.

Maybe that can be of any help. Let me know if you have any questions. The code contains enought comments to follow.

Kind regards,
Daniel