[effects/dimscreen] Make it work on Wayland

Summary:
There is no concept of managed windows in Wayland, so every time we call
managed() on a Wayland client, it will return false. We need to call
that method only for X11 clients.

The resource name part for authentication agents is empty because
KWayland can't get their executable paths.

CCBUG: 398100

Test Plan:
Before:

After:

Reviewers: KWin, davidedmundson

Reviewed By: KWin, davidedmundson

Subscribers: davidedmundson, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D16317