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