fix the outline view's toggle button

Fix #3464, the outline view's toggle button should only be enabled for file types that support an outline view.

@ussuri, I have a hard-coded reference to the toggle button from ace.dart. At some point, I want to move to having the button itself listen for enablement of the 'toggle-outline' command.
`SparkButton` already inherits `SparkWidget.setAttr`, which is 95% of what you're doing here, and shared between all the widgets. Could you use that here? The only difference is that `setAttr` doesn't make the `shouldAdd` parameter optional.