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