Replace MinimumComboBox with regular QComboBox

Summary:
The size constraint can be set externally, we don't need to have a dedicated
widget for that.

Test Plan: Looks and behaves the same as before.

Reviewers: mlaurent

Reviewed By: mlaurent

Subscribers: kde-pim

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D20278