Make integration with KScreenLocker optional in WaylandServer

Summary:
In order to start the WaylandServer in kwin_x11 we need to make sure
that WaylandServer does not start the KScreenLocker integration. On
X11 the lock screen is provided by a different application (in Plasma
by ksmserver).

A new init flag is added to WaylandServer to not integrate with
KScreenLocker. Thus the default is still to integrate with KScreenLocker.

All direct usages of KScreenLocker are guarded to not be called if
the screenlocker integration is not present.

Reviewers: Plasma

Subscribers: plasma-devel

Projects: Plasma

Differential Revision: https://phabricator.kde.org/D1481