We want to avoid handling focus events for the private focus window,
otherwise the keyboard grab taken by for example buttons will cause a
spurious FOCUS_OUT/FOCUS_IN on the toplevel.

The code that did this seems to have been lost in the XI2 transition for
GTK3.

This patch reapplies db4a6040 which was
backed out in 18406b7b to give
developers a chance to get their X servers fixed. As we want to get this
bugfix in for 3.4, we need to commit it now.

https://bugzilla.gnome.org/show_bug.cgi?id=657578