Easy - not a piece of cake, for sure, but it is definitely possible.
We were actually planning on releasing a blog post which elaborates in details on how exactly the ControlTemplateEditor in Sitefinity works, and how one can use it in a custom widget.
Until the blog post is ready (we're aiming this, or next week at latest) I believe you might find the attached sample project useful. The sample has been built on top of a Sitefinity Thunder generated widget with designer, so you can try comparing these in the meantime to get an idea of what has been changed.
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