Support tabbing through controls in modals, popovers, and selects

I rebased the work that @thangdinh did, as he will be unable to work on this for a while. Not very familiar with the code, so hoping that someone else is.

@cyril-sf @azirbel @Dylnuge @jpriceAddepar 
See my comment on the thread @azirbel started above, but in addition, we should simplify our disabled css class syntax here to use the same variable the `disabled` state is using.