Persist size of outline view on resizing

@devoncarew: This is asymmetrical with the split view because the corresponding markup is asymmetrical. I'm planning to try again to move the chunk with the outline and its resizing splitter from spark_polymer.html to spark_polymer_ui.html: then the symmetry will be restored.

Also removed an unused method.
It would be nice to have an `onUpdate` getter on SparkSplitter, that just returned `on['update']`. It would make the event easier to discover.