Improve charge capacity/level strings
Summary: In particular, the string "Full design" is ambigious, bordering on nonsensical.
Test Plan:
(Pay no attention to the fact that my battery reports its original capacity as lower than its current charge level. It's a bit... special)
Reviewers: VDG, Plasma, broulik, filipf
Reviewed By: VDG, Plasma, broulik, filipf
Subscribers: plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D23379