Notify effects when the screen gets locked

EffectsHandlerImpl starts to monitor DBus for the screen being locked and
provides this information to the Effect system by allowing them to ask
whether the screen is currently locked and by emitting a signal when the
screen gets locked/unlocked.

This information is needed to ensure that no private data is shown on the
screen. The following effects are adjusted:
* taskbar thumbnails
* thumbnail aside
* mouse mark
* screen shot


FIXED-IN: 4.11
REVIEW: 108670