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 limits the type of interactive control in a sidenav to a link or a button. One idea to make this more flexible would be to replicate jQuery-UI's `:tabbable` selector. http://api.jqueryui.com/tabbable-selector/