Use DockPanel in Flyouts instead of StackPanel.

Using `DockPanel`, instead of `StackPanel`, for `Flyout`s.  This allows content to scale on resize (e.g. `ScrollViewers`), it also allows for forcing content to the bottom of the `Flyout` -- such as confirm/cancel buttons.

The Flyout demo code was also updated to show how the changes allow a `ScrollViewer` to properly behave.