Improve CSV importer

Summary:
- Make the CSV importer to run as a modal dialog of the application as
other importers do. This avoids a whole bunch of problems.

- Combine the Finish and Import button into a single "Import CSV" button
on the last page of the wizard.

- In case 'Skip setup' is marked on the first page, the wizard stops on
the last page and does not automatically start importing.

BUG: 396759

Test Plan: Tested single file import and web-connect import

Reviewers: wojnilowicz

Reviewed By: wojnilowicz

Differential Revision: https://phabricator.kde.org/D14459