regression from
    commit 306758ab
    Date:   Fri Apr 5 15:40:27 2019 +0200
    tdf#117066 Saving ODT document with ~1500 bookmarks is slow, part 5
Before the above commit, we could have multiple child items with the
same name. Restore that behaviour, while keeping the fast lookup, by
using a std::unordered_map<OUString, std::vector<...

Also remove name from SotElement_Impl so there is no chance of the name
in the key of the map, and the name in the element getting out of sync.

Change-Id: I65c294ddc409d9b8a7006e4f4338c9a2a4446a92
Reviewed-on: https://gerrit.libreoffice.org/72544
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>