KFormat: Allow usage of quantities beyond bytes and seconds

Summary:
Its useful to allow automatic formatting of quantities like bit per
second (bit/s), but formatByteSize can not be used for this. Generalize
the implementation to allow usage for arbitrary quantities.

Test Plan: make test

Reviewers: Frameworks, astippich

Reviewed By: astippich

Subscribers: kossebau, kde-frameworks-devel, astippich

Tags: Frameworks

Maniphest Tasks: T8500

Differential Revision: https://phabricator.kde.org/D13583