Implement Split View Menus

Split View Menus:

* Remove temporary menu items
* Add Split view icon w/tooltip
* Change Split view icon state based on split view scheme
* Create permanent menu
* Localize menu strings
* Fix gear menu so it's always at top of sidebar, add tooltip

cc @JeffryBooher 

Not necessarily. According to CSSLint, you should never, ever use ID selectors. I think it's OK to use ID selectors for elements that will absolutely, never be duplicated, but it's possible someone might want another split view button somewhere, so I think this is OK as a class.

I limited the DOM search to the parent element, and the element is cached:

```js
    $splitViewMenu = $sidebar.find(".working-set-splitview-btn");
```

So I don't think performance is an issue.