Improve layout of CSV export dialog

Summary:
Some parts of the description were unable to read. Removed restrictions
and let the Qt layout system do its job.

Test Plan: Inspect visually

Reviewers: KMyMoney, wojnilowicz

Reviewed By: KMyMoney, wojnilowicz

Subscribers: wojnilowicz

Differential Revision: https://phabricator.kde.org/D4332