Split of settings/migration for images/thumbs DB

This is the first step in really managing the Image metadata database
and the Thumbnail cache database as different entities.

Only changes in the gui are included and as is the code is not
functional, still need to be done:

1) change the structure of digikamrc and it's migration
2) define interaction between widgets in the new migration dialog
3) port and possibly unify "Settings -> database migration" with
"Settings -> configure digikam -> database settings"
4) add a "Backup" button


GUI: some new strings, work in progress