Windows, MSI : User-friendly control of datadir location

Even if documented, the data directory location unobvious in the installer UI.

Which makes some people to complain, e.g here https://mariadb.com/kb/en/controlling-the-database-locations/ , or do weird things, like trying to control datadir via a previously created my.ini (if I understand that post correctly), then complain that service would be using --defaults-file parameter.

To avoid misunderstandings, and make people happier, we should bring datadir location into the instance configuration UI, next to port and root password.