Add tutorial/walkthrough popup option

### What is the problem or limitation you are having?

Currently there is no real way inside an app to create an intuitive walk through of an app. The best option is a window with a slide show almost walk through of how to use the app but that can be confusing or not easy to follow depending on the complexity of the application.

### Describe the solution you'd like

Adding an option to create a walkthrough or tutorial to an app. A good example is the walkthrough for the Glade application for creating GTK layouts. For Gtk this can be done with Popovers, the issue there tho is not all widgets support them. For example Button does not while MenuButton does support popovers. I'm not sure if Popover is the proper or best option, it's just an option I have seen used in other applications.

### Describe alternatives you've considered

Creating a slide show type walk through or tutorial for an app outside of the app.

### Additional context


No implementation files, tests, or entry points are named. Start by reviewing Toga's widget and popover support, then compare the requested behavior with the Glade walkthrough example and the stated Button/MenuButton limitation. Done would require an agreed API and a tested walkthrough mechanism across the relevant native backends.

Written by the indexing model from the issue text.