iron-control-state: add tabindex to hostAttributes

[bendavis78]

I'm assuming the point of iron-control-state is to make an element "focusable". If that's true, then the element must have a tabindex attribute. Otherwise, the browser does not consider the item focusable and won't fire the focus event. Knowing that, shouldn't elements implementing iron-control-state always have a tabindex attribute? Or is this omitted on purpose?

[cdata]

Unfortunately, we cannot do this right away. We have some elements that depend on not getting automatic focusability from IronControlState (iron-autogrow-textarea, paper-input as two examples). However, this seems like a reasonable thing for us to shoot for.

[rslawik]

We cannot have tabindex in hostAttributes due to the difference between no tabindex attribute and tabindex="-1".
See: #76 and #78.
fyi @notwaldorf