Move the outline to the right allowing it to be document-dependant

[tintou]

Problem

The left pane is not really tied to the document context, outline symbols feels out of place there

Proposal

Move the outline symbols to the right

Prior Art

My artistic PR: https://github.com/elementary/code/pull/377

[jeremypw]

I would suggest it be styled as (or even part of) the document view not a sidebar (like the mini-map?).

Needs to be decided what to do in response to the "Toggle sidebar" shortcut. I would suggest it does not hide in response to that -if it is not needed the user can turn the plugin off.