Add command line option no-lockscreen to disable lock screen integration

Summary:
WaylandServer provides the initialization flag to disable lock screen
integration. This couldn't be used yet as there is no command line flag
for it. This change adds a new command line argument and hooks up the
functionality.

The no lock screen integration is useful for the embedded use case where
KWin is just used to render one application.

Reviewers: #kwin

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D17279