Add ability to set active tab from active accordion panel

[simonrolfmore]

By default, accordion panels do not sync with tabs: the correct panel will be shown based on the active tab, but changing the active accordion panel does not change the active tab.

This PR adds that optional feature. I hope it's ok. Please let me know if there are style or implementation issues that you'd like altered!

[reddo]

This works great! Nice work.