custom property's onChanged signal is not shown in the Connections editor

In the attached project, toDoItem has a custom property called "isDeleted"

I want to change this property to true when the trashCanMouseArea filling trashCan is clicked. 
I already made a Connection for clicked() signal in trashCanMouseArea which changes isDeleted to true.

Now I want to make a Connection to the signal onIsDeletedChanged but it's not shown in the Connections editor of toDoItem. Ideally, the user-defined properties' signals should be shown here as well.