[KCMs/Night Color] Make activation criteria and times more obvious

Summary:
This patch polishes the Night Color UI to make it more obvious what the feature does,
what the activation triggers are and mean, and when activation will start and end.

Test Plan:

Reviewers: VDG, Plasma, davidedmundson, romangg, filipf

Reviewed By: VDG, Plasma, davidedmundson, romangg, filipf

Subscribers: filipf, abetts, romangg, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D25920