Add support for `ItemsControl` `Header` and `Footer`

[jeromelaban]

Originally posted by @michael-hawker in #1979 (comment)

We could do a bunch of extra work with a StackPanel and ContentPresenters, but it's super simple to hook-in to the existing properties on the ItemsPresenter for this scenario:

     <ItemsPresenter win:Footer="{TemplateBinding ItemsFooter}"
                     win:Header="{TemplateBinding ItemsHeader}"
                     TabFocusNavigation="Local" />

[michael-hawker]

Thanks @jeromelaban! We stumbled upon this one by accident, we didn't even realize ItemsPresenter had Header and Footer properties to use. But it played really well into our new scenario for the SettingsExpander control and presenting an item at the end of the list. It was super simple to plug-in our new properties too, and worked like a charm in UWP and Windows App SDK:

Appreciate you opening a new tracking issue for this.

[michael-hawker]

FYI, I was just looking at the Toolkit's HeaderedItemsControl and realized that it'll be simpler and better aligned to ListView if we use the Header property of the ItemsPresenter itself vs. our own ContentPresenter. Simplifies the visual tree for this light-weight control as well.