ScreenShield: don't allow events through the lock dialog

Make the lock dialog group reactive, to intercept any events
before they go to the actors below.
In the future, we may restructure our chrome to have a clear
layer system, but for now it fixes a security issue in the lock
screen (you can see the contents of the windows by dragging
if the screen was locked with the overview active)

https://bugzilla.gnome.org/show_bug.cgi?id=705840