[scripting] Emit clientAdded for Wayland clients

Summary:
Currently, if a script relies on clientAdded to setup some required
connections, then it probably won't work with Wayland clients because
clientAdded is emitted only for X11 clients.

Test Plan:
* Installed sticky window snapping script, it works;
* Installed a couple of tiling scripts, with some small changes, they work.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: strobach, TomButler, davidedmundson, mart, graesslin, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D17097