thus we reimplement it for Qt5DropTarget. This is qt5 remix of
tdf#118302 (in Calc drop into the same tab should cancel DnD, instead
of causing data loss)

Change-Id: Ib37ea5a018133779e85e8e131d81bb6cee7d9206
Reviewed-on: https://gerrit.libreoffice.org/71531
Tested-by: Jenkins
Reviewed-by: Katarina Behrens <Katarina.Behrens@cib.de>