Basically I had the idea by myself but then I've seen
David Stripinis' cleanPolyCombine from 1999 needed
an update (didn't work anymore) ;] Features:
* it combines without leaving empty groups (which occur if secondary objs have helper nodes parented)
* it keeps the new combined mesh in the group of
  the first object in the selection, not in root
* it renames the new object after the first selected
  object
* it puts the pivot where the 1st object had its
* it reorders the final object to where the 1st was
* it preserves the firsts object rotation
* reataches animation and any other incoming/outgoing connections
* reparents first objects constraints