Emit Workspace::clientAdded() with AbstractClient

Summary:
Currently, if one wants to monitor for new X11 and Wayland clients, they
have to listen for two signals. Workspace::clientAdded() is emitted only
for X11 and Xwayland clients, while WaylandServer::shellClientAdded() is
emitted only for xdg-shell clients. Such design doesn't scale well.

As the first towards emitting Workspace::clientAdded() for new Wayland
clients, this change replaces the X11Client parameter in the clientAdded
signal with AbstractClient.

Test Plan: Tests still pass.

Reviewers: KWin, cblack

Reviewed By: cblack

Subscribers: kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D29665