Re: How to create a custom sorting properties in Qlik Sense Extension?
I think I have good news for you, we are about to publish something in Qlik Sense 2.2 which is probably what you are describing. Follow the changes regarding which components can be used in Qlik Sense visualization extensions.
The API this visualization extension is using is not documented (getting that list in the property panel) but actually already there fore quite some while. So you can use it, but it is not officially supported. The latter will change with Qlik Sense 2.2.