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 

@larz0 This fixes problem with [bg image disappearing when you hold down the mouse key](https://github.com/adobe/brackets/pull/9109#issuecomment-55793345). I'm not sure what this rule is for, so let me know if you think it might break something (and what to test).