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 

@JeffryBooher It's probably more appropriate for "working-set-option-btn" and "working-set-splitview-btn" to be in the "working-set-list-container" container, but since the contents for that element are auto-generated, maybe that should be wrapped in a "working-sets" element? Those buttons apply to all working sets, so it doesn't make sense to control them in WorkingSetView.js. Maybe MainViewManager should control them?