Summary:
Format the amount of available RAM using KFormat, but keeping the same
unit, and scale (since the controls there refer to specific ones).

Test Plan:
The amount of RAM is shown in KiB as before, although
formatted according to KFormat & locale.

Reviewers: #krita, rempt

Reviewed By: #krita, rempt

Tags: #krita

Differential Revision: https://phabricator.kde.org/D13775