Revert the 2.0 decision to always use KB for file-sizes

The feedback on bugs.kde.org has shown that the previous behavior
(= show size with best-matching unit) is preferred by most users.
I initially wanted to make this configurable, but for implementing
it in a non-hacky way extending KLocale from kdelibs would have
been required. I'm not sure whether the usecase in Dolphin justifies
having such a configuration in KLocale - however as kdelibs is frozen
at the moment this is no option and the old behavior has been
restored.

BUG: 289850
FIXED-IN: 4.8.3