flags are stored by ScTransferObj and ScDrawTransferObj
in members nDragSourceFlags, implemented in the same way by both classes.
set only in ScTabControl::DoDrag() and ScContentTree::DoDrag().

convert to typed_flags ScDragSrc and
move flags closer to users, from sc/inc/scmod.hxx to
sc/source/ui/inc/viewdata.hxx

Change-Id: I15f58a5c828e154114971a1fcf941a3e27acbd56
Reviewed-on: https://gerrit.libreoffice.org/27303Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Noel Grandin <noelgrandin@gmail.com>