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 

Forgot to mention that we need to add drop-shadow for the icons to match the gear icon. Use this value:

```
.working-set-splitview-btn {
…
-webkit-filter: drop-shadow(0 1px 0 rgba(0,0,0,0.36));
}
```