CustomMultiChildLayout and CustomOneChildLayout now use their delegate's shouldRelayout() method instead of a "token" to decide if layout is needed.

MultiChildLayoutDelegate and OnChildLayoutDelegate are now expected to be stateless, i.e. they'll typically be built each time their custom layout widget is built. If the identical layout delegate is provided to a new custom layout, layout will not happen.

Revised the bottom sheet implementation per the new custom layout classes. Removed a SizeObserver.

Fixes #899