Summary:
Currently each managed X11 client is represented with an instance of
Client class, however the name of that class is very generic and the
only reason why it's called that way is because historically kwin
was created as an x11 window manager, so "Client" was a sensible choice.

With introduction of wayland support, things had changed and therefore
Client needs to be renamed to X11Client in order to better reflect what
that class stands for.

Renaming of Client to X11Client was agreed upon during the last KWin
sprint.

Test Plan: Compiles, the test suite is still green.

Reviewers: #kwin, romangg

Reviewed By: #kwin, romangg

Subscribers: romangg, davidedmundson, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D24184