[KCM/Night Color] Use a spinbox with an appropriate suffix for the duration
Summary:
Currently it's a text box that doesn't give you any indication of what the units are.
BUG:413249
FIXED-IN: 5.18.0
Test Plan:
Reviewers: VDG, Plasma, davidedmundson
Reviewed By: Plasma, davidedmundson
Subscribers: davidedmundson, plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D24813