All you need for the following exercises are
Qlik Sense Desktop 3.x or
Qlik Sense Server 3.x
Open Dev Hub
In the hub go to the menu and open Dev Hub:
Note: In case of Qlik Sense Desktop a new browser window will be opened, minimize the instance of Qlik Sense Desktop while working in the newly opened browser window.
In Dev Hub you will not only see a new link pointing you to the Widget editor, but also all existing Widget libraries will be shown: