Interactive background views

Scenario: you‘ve added a background image to your App Bar and you’d now like to be able to tap the background image.

This is not trivial to do with the App Bar APIs due to considerations being discussed in Issue #184.

The heart of the limitation is that we're using a view (headerStackView) to lay out the Navigation Bar. If you add a background view behind the headerStackView instance then headerStackView will end up eating all of your touch events.

Until Issue #184 is resolved, our recommendation for building interactive background views is the following:

1. Do not use the App Bar component.
2. Create your own Flexible Header. Learn more by reading the Flexible Header Usage docs.
3. Add your views to this flexible header instance.
4. Create a Navigation Bar if you need one. Treat it like any other custom view.