[KCMs/Night Color] Add a valueForText handler for the new SpinBox
Summary: Now you can enter "1 minute" and it will be parsed correctly
Test Plan: Enter "1 minute" and tab away from the box
Reviewers: Plasma, zzag
Reviewed By: Plasma, zzag
Subscribers: zzag, plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D24838