Use a lock file to indicate the screen is locked

The lock file is touched when the screen gets locked. If the
file cannot be created the screen won't lock. If there is the
lock file when KWin starts up, the screen locker will be activated.
This ensures that KWin will safely lock the screen even after a
crash.

The lock file is set in the cache standard dir. This should be
moved to the run standard dir after it has been implemented.
The lock file is owned by the user and not writable by group or
others. This means only the user can remove the lock file and by
that end the screen locker when KWin crashes/restarts. As it
requires a local access it should be save enough. It might be
possible to make it owned by root and use something like polkit
to provide the security, but that is something someone more
competent in that area has to decide. The previous implementation
allowed to end the screen locker by killing a process run by the
current user, so it is as secure as the previous implementation.