Add applet for controlling Night Color
Summary:
The new applet allows to temporarily disable Night Color, for example if
one wants to edit a video or an image.
BUG: 400418
Reviewers: Plasma, VDG, ngraham, davidedmundson
Reviewed By: Plasma, VDG, ngraham, davidedmundson
Subscribers: davidedmundson, ngraham, plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D25947