[Energy] Put battery capacity info in battery section and call it "Capacity degradation"
Alternative to D22882.
Our use of "capacity" to refer to how much of a battery's original capacity remains is
confusing. This is made less confusing by inverting the number and referring to it as
"degradation". "Capacity degradation: 21%" communicates that the battery has lost
21% of its original capacity far better than "Capacity: 79%" does, because the latter
is easily confused with the battery's current charge level.
To further reduce the confusion, the battery's degradation level is relocated to end
of the Battery section.
Reviewed By: VDG, filipf
Subscribers: aspotashev, filipf, plasma-devel
Differential Revision: https://phabricator.kde.org/D23270