refactor the mega-widget dnd drop target to be reused at
mini-widget level

and weld SwInsertBookmarkDlg

Change-Id: I3f36e1cd902ecf166e369d0a05902d3df2b4f791
Reviewed-on: https://gerrit.libreoffice.org/69274
Tested-by: Jenkins
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>