[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: {F7639735, size=full}

Reviewers: #vdg, #plasma, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: davidedmundson, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D24813