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.
Custom event discoverability is a general and not completely solved problem. I raised it on the Dart and JS Polymer boards at some point, proposing `custom-events` attribute in an element's declaration, akin to `attributes`:
https://groups.google.com/a/dartlang.org/forum/#!mydiscussions/web/kZ8SiS9DXRc
https://groups.google.com/forum/#!topic/polymer-dev/5Vk1Fnooyeo

As you can see, Polymer JS is pursuing a pure documentation approach, but Polymer Dart is more open to the idea. I think the documentation approach will ultimately fail, because it's unmaintainable in the long run.

I also have a TODO to implement and enforce `custom-events` at least for Spark widgets.

Regarding `onUpdate`: it would be helpful on the Dart side, but much less so with the primary Polymer use case, which is attaching a listener to the custom event directly in the HTML. `Outline` and its resizer are moving to `<spark-polymer-ui>` very soon, which means they'll switch to that primary use case.