QML screen locker using KGreeter

ScreenLocker uses a QML scene with the wigdets provided by the
KGreeterPlugins embedded as proxy widgets (yes that sucks, but
it's the best we get without breaking everything).

When clicking the unlock button in the QML scene a signal is emitted
which triggers the greeter verify process. If it succeeds the lock
screen is removed. Failure is not yet supported (restart of greeter).

Currently there are the following remaining issues:
* bad layout of proxy widget
* proxy widget is not auto-focues
* return key does not accept
* switch user not yet implemented
* messages from greeter not passed to view
* authentication failure not yet supported