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
Merge pull request #945 from ahocevar/select-followup

Follow-up improvements for #897