overview: Ignore page changes when overview is not open
Otherwise we might end up focusing the wrong app when another adds a new toplevel on startup (like e.g. calls).
I have a bigger rework in the works but this should do the trick for 0.42 so we can finally have calls, chatty and other apps using a daemon spawn quickly.
See GNOME/calls!760 for a related calls MR.
Edited by Guido Günther