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)

Ok, I'll just list my sources of worrying here and let you decide:

1. With this, we're adding yet one more way of handling UI events to the half-dozen that we're already have.
2. If every piece of code starts adding it's own listeners to random events, soon we'll lose sight of who's handling what, in what order, and whether they're randomly blocking events from each other with `stopPropagation`, as here. Perhaps somebody else in the app would like to know when the outline gets toggled? EventBus would provide a single point to look at and debug, and ensure an orderly event propagation.
3. Again, this goes against a clean separation of UI and business logic.
4. We're hard-coding 'command' and 'toggle-outline' here.