SelectSizeTypeComboBox: Use KiB/MiB/GiB quantifiers for clarity

Summary:
Binary SI units are used in Sieve, see
https://tools.ietf.org/html/rfc5228#section-2.4.1

Test Plan: it compiles

Reviewers: kde-pim, winterz

Reviewed By: winterz

Subscribers: winterz, kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D26025