Check that the application is responding when activating a window

Currently, we ping windows only when attempting to delete them, but
if the application is not responding, we want to show the dialog
as soon as possible. Given that we cannot be passively notified that
the window stopped responding with the current X11 protocol, a good
workaround is to ping the window when activating it.
If the window stops responding while active, it is expected the user
will try to switch window or open the overview, and when coming back
he'll get the failure dialog.

https://bugzilla.gnome.org/show_bug.cgi?id=684340