Changing layer value should trigger resort of parent's children

Right now `layer` is respected when a positionable is added to a `View` or a `Positionable`. But if the value of `layer` changes, the collection is not re-sorted.

The `layer` field should be made private and `get` and `set` accessors added. The `set` accessor should remove and re-add the positionable so it is re-sorted in its parent's collection.