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.
This check iterates through every control in a sidenav and changes its' `tabIndex`. If there is a better way to accomplish this for performance, I'm all ears. One alternative would be to set the parent sidenav element to `display: none` after it is done animating.