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)

The placement is odd:

1. The button now separates tabs from the navigation buttons, which have a pretty strong relationship to tabs.
2. The button will now idle a lot of the time, especially when a developer doesn't do much Dart.
3. The relationship between the button and the Outline view is now unclear. Especially since we start with the Outline view on by default: how will a user guess that they can hide the view by clicking this button?
4. This doesn't play well with the markdown preview button.