[effects/dimscreen] Use QSet for checking whether activated window asks for permissions

Summary:
Do not construct QStringList with classes of windows that ask for
permissions. Instead, create a static set of those window classes (to
avoid the unnecessary construction of QStringList and make lookups
faster).

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D13440