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 

The working set context menu was moved back to `WorkingSetView`, but `SidebarView` now owns "working-set-option-btn" so there is only a single copy and it doesn't jump around.