Load two sheets, first with grp_refs to loclib; use the buffer to copy these
grp_refs to the second sheet:
  -> broken ref on the target buffer; three possible solutions:

Possible solutions:

1. on copy: convert group ref to group on copy and mark loclib-enable
   -> BAD: this will break for non-loclib grp_refs

2. on paste: remember group ref sheet on copy and if on paste there's a
   mismatch, do a full copy
   -> BAD: this will break if the source sheet is closed while grp_refs
      are in buffer already!

3. buffer could have an indirect subtree too, copy would fill it in
   - on copy create indirect symlib in buffer and copy the loclib entry
   - on paste compare indirects to the sheet's indirect hash, insert if needed
   - all this done by loclib code; non-loclib grp_refs should be refused
