[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