Flatten the vector of SfxPoolItemArray_Impl, to reduce pointer chasing.
This struct is movable, etc, so no need to allocate it separately on the
heap.

Change-Id: I794b4356660e9cd0e63bc98b011f58162a838662
Reviewed-on: https://gerrit.libreoffice.org/70884
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>