[effects/diminactive] Dim Wayland clients

Summary:
There is no concept of managed windows in Wayland, so every time we call
managed() on a Wayland client, it will return false.

This change addresses that problem by invoking managed() only for X11 clients.

CCBUG: 398100

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D16312