[nightcolor] Introduce inhibition API

Summary:
The new API allows to block Night Color temporarily. This can be useful
for applications such as video games and plasma applets.

CCBUG: 400418

Test Plan: Called inhibit() and uninhibit() from D-Feet.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D25786