[KCM/NightColor] Allow the user to know the default settings are selected

Test Plan:
kcmshell5 kcm_nightcolor
Checked all settings reacts correctly to the restore defaults button

Reviewers: #kwin, crossi, ervin, romangg, davidedmundson

Reviewed By: #kwin, romangg, davidedmundson

Subscribers: davidedmundson, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D26049