WIP: Improve accessibility for Material Sidenav

To improve the experience for keyboard users with smaller monitors, links and buttons in hidden `material-sidenav` components should not be part of the tab order. That way users cannot tab onto controls that are visually obstructed.

WIP: Keyboard focus should be managed for the "Toggle Left" / "Toggle Right" buttons, where togging from a button sends focus into the corresponding sidenav.
Note: This causes a bug in the expanded desktop layout where the sidenav links/buttons are removed from the tab order.