move the toggle outline button

A few changes:

- give spark-buttons the ability to fire 'command' events. When their `command` attribute is set, a mouse click to cause them to fire a custom `command` event into the dom
- move the outline toggle button from hovering in the text area into the toolbar. This is in support of eventually making the outline view a separate component to the right of the editing area, rather than something that hovers over the editing area.

@ussuri 

![screen shot 2014-09-18 at 3 44 22 pm](https://cloud.githubusercontent.com/assets/1269969/4328506/5b203486-3f86-11e4-8dae-07941194e650.png)

I'd really prefer to not mix the UI with the business logic like this. Could you intercept the custom event in spark_polymer_ui.dart, just like the menu selections are intercepted in `onMenuSelected`, and either use an explicit new `Action` to toggle the Outline, or at least put a CDE event into the EventBus and listen to it here?