Summary:
Such an option would be useful for people that are used to watch videos
in the fullscreen mode.

By default, we still dim fullscreen windows because watching videos in
fullscreen mode is not the only one use-case. One could have a text
editor in fullscreen mode on one screen and Konsole on another screen.
In that case, it would be desired to dim the text editor if the latter
is active. Also, because we don't have stats of how the fullscreen mode
is used by KDE Plasma users.

BUG: 399822
FIXED-IN: 5.15.0

Test Plan:
* Unchecked the "Fullscreen windows" checkbox;
* Opened Konsole;
* Opened Firefox in the fullscreen mode;
* Pressed Alt+Tab;
* (Firefox stayed bright)

Reviewers: #kwin, #plasma, davidedmundson

Reviewed By: #kwin, #plasma, davidedmundson

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D16216