* add floatingPanel(_:contentOffsetForPinning:)
* add 'Show NavigationController' sample
* fix the initial content offset in a navigation bar with a large text
The content offset preservation should be applied only when
`FloatingPanelController.contentInsetAdjustmentBehavior` is `.always`.
This is because the library user loses control of the initial offset.
- `.full` position's height must be the intrinsic height.
- Work it for a safe area bottom anchor
- Remove FloatingPanelIntrinsicLayout.contentViewController because it
isn't actually needed
And add a sample code to test show(_:sender:) in ContentVC.
`FloatingPanelController.show(_:sender:)` can block 'Show' segue in a
content view controller. 'Show' segue should not be handled by
'FloatingPanelController`. So I replace this method.