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.
Just out of curiosity, have you tried `@published_reflected` here? Regardless, please add `// TODO(ussuri): BUG #2252` before this?