How to create a custom control to add a list of groups with a custom application? specifically for use in the widget configuration?

Yes , you can start creating a custom control using the link you mentioned , thata the latest way of creating a custom control .

Before you do please check the existing available control which are available already out of the box here  Property Templates  

Thank you for the response.. I am at the point now, where I need to create the list of custom applications... so your answer is helpful, but I need to get into deeper.. so here it goes:

I need to create this control for a  list of a custom application (Map Apps), just like the blog widget settings Custom list.. the custom item.

when custom is select it will then create the below control: 

I am setting up the api for it now and making sure I can get the property custom group first.. which will not do anything until i figure out the control here..
next post, I will post the code examples to build it out.