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 

Can't `WorkingSetView` handle its own context menus?  Seems like we should go digging into DOM elements and listening for events on them -- The view that creates them should be the only thing that really knows about them.