Expose `ITabbableContainer`
After upstream fixes in FxCop, having `ITabbableContainer` be `internal`
caused consumer-side failures of the CA2119 inspection. That is because
all client-side inheritors of `TabbableContainer` (and transitively,
`TextBox`), could override `bool CanBeTabbedTo { get; }`, which was
defined on previously-internal `ITabbableContainer`, triggering the
inspection.