Support starting kwin_wayland without XWayland support

Summary:
Just continues with the steps without starting XWayland.

This does not yet fully support kwin_wayland without X11, it currently
still crashes on teardown.

Test Plan: kwin_wayland --socket=wayland-1 kwrite (note the missing --xwayland)

Reviewers: KWin, Plasma

Subscribers: plasma-devel, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D7924