2034 optimize sidebar structure for keyboard support (#2165)

* #2043: add keyboard support to WindowSidebarButtons

* #2034: add keyboard support to WindowsSideBarButtons

* #2034: adds focus handling

* refactors the arrow key navigation

* cosmetics

* #2034: fix build

* #2034 refactoring

* #2034: fixes tests

* #2034 cosmetics

* add missing semicolon

* cosmetics

* remove console.debug()

* Update the WindowSideBar focus implementation to use refs and ReactDOM.findDOMNode instead of document.querySelectorAll (which allows us to test a bit easier) (#2284)

* re-add tests

* fix build