Improve charge capacity/level strings

Summary: In particular, the string "Full design" is ambigious, bordering on nonsensical.

Test Plan:
{F7274016, size=full}
(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