x11: Avoid spurious focus events on grabs

x11: Avoid spurious focus events on grabs

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 db4a6040afd3d1cabbb306037e811f37d62e2e76 which was
backed out in 18406b7b04da088f8a3efe7cca5ac1647080dc0b 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