Follow-up improvements for #897

* Interaction is no goog.Disposable any more.
* Permanent cleanup during selection instead of disposeInternal.
* Moved selectionLayers creation outside feature loop.
* Maintain selectedFeatures and unselectedFeatures only for
  layers that have a setRenderIntent method.
instanceof check to avoid ducktyping
Do not delete from oldFeatureMap
Storing selection layer and selected features on the layer

With this change, other interactions that rely on selection can
get along without knowing about the Select interaction.
Removing no longer needed getFeatureWithUid method.