some hard to reliably reproduce crashes when drag'n'dropping slides
in slide sorter in Impress can be tracked down to null drop target.
Not every SalFrame is registered as drop target, so let's accept
drops (QWidget::setAcceptDrops) only for those frames that are.

Change-Id: I01f006d619209c558e8d9976116daad65f51d7d9
Reviewed-on: https://gerrit.libreoffice.org/71533
Tested-by: Jenkins
Reviewed-by: Katarina Behrens <Katarina.Behrens@cib.de>