[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