Use Migration warn dialog directly in library dba instead of popping up
window in dbu and passing down the information through XDataSource.

Change-Id: I6b8d2b55408a741662052d4af316b0c662d799f0
Reviewed-on: https://gerrit.libreoffice.org/53363Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Tamás Bunth <btomi96@gmail.com>