Use KFormat for formatting byte sizes
Summary:
Use what is already available in KCoreAddons, instead of two custom
versions of it.
Test Plan: Builds fine, and the standard formatting is used.
Reviewers: Krita, dkazakov
Reviewed By: Krita, dkazakov
Subscribers: dkazakov, alvinhochun
Tags: Krita
Differential Revision: https://phabricator.kde.org/D13548