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'm deliberately using the DOM for dispatching here. Depending on where the button is in the DOM hierarchy, it's command event might be handled by different consumers. So, if there were two separate editing areas, with different outline views, the correct outline would be toggled when toggle button is hit based on their DOM relationship.

The event bus would also achieve be decoupled (but without the hierarchical feature). I'd like to try using the DOM for messaging for a bit and see how that works out.