Warn user with a pop-up dialog before migration.

To do that we have to know the database URL at UI level. In order to get
that I updated XDataSource interface with getConnectionURL().

The dialog offers two options: proceed with or without migration. If the
user choose "yes", we store that information in XDataSource. The
migration itself will be made in buildLowConnection().

Change-Id: I1f0d03da6352f7a0a8d989da79c4b2fe60a03ca1
Reviewed-on: https://gerrit.libreoffice.org/52876Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Tamás Bunth <btomi96@gmail.com>