Use field group label in h2 above vertical tabs group

This is a follow up issue to #1155246: Use field group label in h2 above horizontal tabs group → . I also needed to have label on top of vertical tabs.

I appreciate this one is trickier because vertical tabs is inside core but I have two requests, one that allows themer functions to do this in the same way as the patch in the horizontal issue, and one suggested addition that overrides the core's theme function to add support for the group label.

I think the first should really be accepted while the latter one I leave you to decide.