Always highlight the nearest outline item

[jiewangjs]

Thanks to the author for such a nice extension.
As I have suggested in the title, right now, when viewport has no outline item, the highlight on the outline map lost, it is very easy to lose track.
I ponder this feature won't be hard, hope this can be added.

[Gerrnperl]

Thanks for your suggestion :). I'm sorry I almost forgotten this point, and now the extension will try to focus on the closest item to the selection area when no symbol is selected.