compute the width of the track import label to adjust its box

Summary:
should help to ensure that localized interfaces are still fine looking

BUG: 398924

Test Plan: the box width gets adjusted if the text to shown is quite big

Reviewers: astippich, januz, aspotashev

Reviewed By: aspotashev

Subscribers: aspotashev

Differential Revision: https://phabricator.kde.org/D15779