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.
Since this is (very?) temporary, it's ok. Otherwise I'd complain :)