Fix spark-splitter

Some recent change in Polymer has changed the way custom elements get registered. As a result of that, and possibly some wrong Polymer usage in Spark, all Xyz.created ctors for Spark widgets were called twice. Calling real initialization from there for SparkSplitter caused a crash on the second call, because _target would be initialized to null for some reason. Redone to use enteredView(), which is probably more correct anyway.

TBR: @terrylucas